add_filter - Documentation

WordPress Version: 5.8

/**
 * Adds a callback function to a filter hook.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *         return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Bound callbacks can accept from none to the total number of arguments passed as parameters
 * in the corresponding apply_filters() call.
 *
 * In other words, if an apply_filters() call passes four total arguments, callbacks bound to
 * it can accept none (the same as 1) of the arguments or up to four. The important part is that
 * the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
 * opted to accept. If no arguments were accepted by the callback that is considered to be the
 * same as accepting 1 argument. For example:
 *
 *     // Filter call.
 *     $value = apply_filters( 'hook', $value, $arg2, $arg3 );
 *
 *     // Accepting zero/one arguments.
 *     function example_callback() {
 *         ...
 *         return 'some value';
 *     }
 *     add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.
 *
 *     // Accepting two arguments (three possible).
 *     function example_callback( $value, $arg2 ) {
 *         ...
 *         return $maybe_modified_value;
 *     }
 *     add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.
 *
 * *Note:* The function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes, so
 * everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global WP_Hook[] $wp_filter A multidimensional array of all hooks and the callbacks hooked to them.
 *
 * @param string   $hook_name     The name of the filter to add the callback to.
 * @param callable $callback      The callback to be run when the filter is applied.
 * @param int      $priority      Optional. Used to specify the order in which the functions
 *                                associated with a particular filter are executed.
 *                                Lower numbers correspond with earlier execution,
 *                                and functions with the same priority are executed
 *                                in the order in which they were added to the filter. Default 10.
 * @param int      $accepted_args Optional. The number of arguments the function accepts. Default 1.
 * @return true Always returns true.
 */
function add_filter($hook_name, $callback, $priority = 10, $accepted_args = 1)
{
    global $wp_filter;
    if (!isset($wp_filter[$hook_name])) {
        $wp_filter[$hook_name] = new WP_Hook();
    }
    $wp_filter[$hook_name]->add_filter($hook_name, $callback, $priority, $accepted_args);
    return true;
}

WordPress Version: 5.6

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *         return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Bound callbacks can accept from none to the total number of arguments passed as parameters
 * in the corresponding apply_filters() call.
 *
 * In other words, if an apply_filters() call passes four total arguments, callbacks bound to
 * it can accept none (the same as 1) of the arguments or up to four. The important part is that
 * the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
 * opted to accept. If no arguments were accepted by the callback that is considered to be the
 * same as accepting 1 argument. For example:
 *
 *     // Filter call.
 *     $value = apply_filters( 'hook', $value, $arg2, $arg3 );
 *
 *     // Accepting zero/one arguments.
 *     function example_callback() {
 *         ...
 *         return 'some value';
 *     }
 *     add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.
 *
 *     // Accepting two arguments (three possible).
 *     function example_callback( $value, $arg2 ) {
 *         ...
 *         return $maybe_modified_value;
 *     }
 *     add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.
 *
 * *Note:* The function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes, so
 * everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global WP_Hook[] $wp_filter A multidimensional array of all hooks and the callbacks hooked to them.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callable $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action. Default 10.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter;
    if (!isset($wp_filter[$tag])) {
        $wp_filter[$tag] = new WP_Hook();
    }
    $wp_filter[$tag]->add_filter($tag, $function_to_add, $priority, $accepted_args);
    return true;
}

WordPress Version: 5.4

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *         return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Bound callbacks can accept from none to the total number of arguments passed as parameters
 * in the corresponding apply_filters() call.
 *
 * In other words, if an apply_filters() call passes four total arguments, callbacks bound to
 * it can accept none (the same as 1) of the arguments or up to four. The important part is that
 * the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
 * opted to accept. If no arguments were accepted by the callback that is considered to be the
 * same as accepting 1 argument. For example:
 *
 *     // Filter call.
 *     $value = apply_filters( 'hook', $value, $arg2, $arg3 );
 *
 *     // Accepting zero/one arguments.
 *     function example_callback() {
 *         ...
 *         return 'some value';
 *     }
 *     add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.
 *
 *     // Accepting two arguments (three possible).
 *     function example_callback( $value, $arg2 ) {
 *         ...
 *         return $maybe_modified_value;
 *     }
 *     add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.
 *
 * *Note:* The function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes, so
 * everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter A multidimensional array of all hooks and the callbacks hooked to them.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callable $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action. Default 10.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter;
    if (!isset($wp_filter[$tag])) {
        $wp_filter[$tag] = new WP_Hook();
    }
    $wp_filter[$tag]->add_filter($tag, $function_to_add, $priority, $accepted_args);
    return true;
}

WordPress Version: 4.7

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *         return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Bound callbacks can accept from none to the total number of arguments passed as parameters
 * in the corresponding apply_filters() call.
 *
 * In other words, if an apply_filters() call passes four total arguments, callbacks bound to
 * it can accept none (the same as 1) of the arguments or up to four. The important part is that
 * the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
 * opted to accept. If no arguments were accepted by the callback that is considered to be the
 * same as accepting 1 argument. For example:
 *
 *     // Filter call.
 *     $value = apply_filters( 'hook', $value, $arg2, $arg3 );
 *
 *     // Accepting zero/one arguments.
 *     function example_callback() {
 *         ...
 *         return 'some value';
 *     }
 *     add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.
 *
 *     // Accepting two arguments (three possible).
 *     function example_callback( $value, $arg2 ) {
 *         ...
 *         return $maybe_modified_value;
 *     }
 *     add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.
 *
 * *Note:* The function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes, so
 * everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callable $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter;
    if (!isset($wp_filter[$tag])) {
        $wp_filter[$tag] = new WP_Hook();
    }
    $wp_filter[$tag]->add_filter($tag, $function_to_add, $priority, $accepted_args);
    return true;
}

WordPress Version: 4.5

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *         return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Bound callbacks can accept from none to the total number of arguments passed as parameters
 * in the corresponding apply_filters() call.
 *
 * In other words, if an apply_filters() call passes four total arguments, callbacks bound to
 * it can accept none (the same as 1) of the arguments or up to four. The important part is that
 * the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
 * opted to accept. If no arguments were accepted by the callback that is considered to be the
 * same as accepting 1 argument. For example:
 *
 *     // Filter call.
 *     $value = apply_filters( 'hook', $value, $arg2, $arg3 );
 *
 *     // Accepting zero/one arguments.
 *     function example_callback() {
 *         ...
 *         return 'some value';
 *     }
 *     add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.
 *
 *     // Accepting two arguments (three possible).
 *     function example_callback( $value, $arg2 ) {
 *         ...
 *         return $maybe_modified_value;
 *     }
 *     add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.
 *
 * *Note:* The function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes, so
 * everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added,
 *                               it doesn't need to run through that process.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callable $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}

WordPress Version: 4.4

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *     	   return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Bound callbacks can accept from none to the total number of arguments passed as parameters
 * in the corresponding apply_filters() call.
 *
 * In other words, if an apply_filters() call passes four total arguments, callbacks bound to
 * it can accept none (the same as 1) of the arguments or up to four. The important part is that
 * the `$accepted_args` value must reflect the number of arguments the bound callback *actually*
 * opted to accept. If no arguments were accepted by the callback that is considered to be the
 * same as accepting 1 argument. For example:
 *
 *     // Filter call.
 *     $value = apply_filters( 'hook', $value, $arg2, $arg3 );
 *
 *     // Accepting zero/one arguments.
 *     function example_callback() {
 *         ...
 *         return 'some value';
 *     }
 *     add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.
 *
 *     // Accepting two arguments (three possible).
 *     function example_callback( $value, $arg2 ) {
 *         ...
 *         return $maybe_modified_value;
 *     }
 *     add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.
 *
 * *Note:* The function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes, so
 * everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added,
 *                               it doesn't need to run through that process.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callable $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}

WordPress Version: 4.3

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *     	   return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Since WordPress 1.5.1, bound callbacks can take as many arguments as are
 * passed as parameters in the corresponding apply_filters() call. The `$accepted_args`
 * parameter allows for calling functions only when the number of args match.
 *
 * *Note:* the function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes,
 * so everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added,
 *                               it doesn't need to run through that process.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callback $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}

WordPress Version: 4.1

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 *
 * Note that `$example` is passed to the callback, (maybe) modified, then returned:
 *
 *     function example_callback( $example ) {
 *         // Maybe modify $example in some way.
 *     	   return $example;
 *     }
 *     add_filter( 'example_filter', 'example_callback' );
 *
 * Since WordPress 1.5.1, bound callbacks can take as many arguments as are
 * passed as parameters in the corresponding apply_filters() call. The `$accepted_args`
 * parameter allows for calling functions only when the number of args match.
 *
 * *Note:* the function will return true whether or not the callback is valid.
 * It is up to you to take care. This is done for optimization purposes,
 * so everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added,
 *                               it doesn't need to run through that process.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callback $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return boolean true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}

WordPress Version: 4.0

/**
 * Hook a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 * Note that $example is passed to the callback, (maybe) modified, then returned:
 *
 * <code>
 * function example_callback( $example ) {
 * 	// Maybe modify $example in some way
 * 	return $example;
 * }
 * add_filter( 'example_filter', 'example_callback' );
 * </code>
 *
 * Since WordPress 1.5.1, bound callbacks can take as many arguments as are
 * passed as parameters in the corresponding apply_filters() call. The $accepted_args
 * parameter allows for calling functions only when the number of args match.
 *
 * <strong>Note:</strong> the function will return true whether or not the callback
 * is valid. It is up to you to take care. This is done for optimization purposes,
 * so everything is as quick as possible.
 *
 * @since 0.71
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added,
 *                               it doesn't need to run through that process.
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callback $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return boolean true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}

WordPress Version: 3.9

/**
 * Hooks a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 * Note that $example is passed to the callback, (maybe) modified, then returned:
 *
 * <code>
 * function example_callback( $example ) {
 * 	// Maybe modify $example in some way
 * 	return $example;
 * }
 * add_filter( 'example_filter', 'example_callback' );
 * </code>
 *
 * Since WordPress 1.5.1, bound callbacks can take as many arguments as are
 * passed as parameters in the corresponding apply_filters() call. The $accepted_args
 * parameter allows for calling functions only when the number of args match.
 *
 * <strong>Note:</strong> the function will return true whether or not the callback
 * is valid. It is up to you to take care. This is done for optimization purposes,
 * so everything is as quick as possible.
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added, it doesn't need to run through that process.
 *
 * @since 0.71
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callback $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        (optional) The order in which the functions associated with a particular action are executed. Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the action.
 *                                  Default 10.
 * @param int      $accepted_args   (optional) The number of arguments the function accepts.
 *                                  Default 1.
 * @return boolean true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}

WordPress Version: 3.7

/**
 * Hooks a function or method to a specific filter action.
 *
 * WordPress offers filter hooks to allow plugins to modify
 * various types of internal data at runtime.
 *
 * A plugin can modify data by binding a callback to a filter hook. When the filter
 * is later applied, each bound callback is run in order of priority, and given
 * the opportunity to modify a value by returning a new value.
 *
 * The following example shows how a callback function is bound to a filter hook.
 * Note that $example is passed to the callback, (maybe) modified, then returned:
 *
 * <code>
 * function example_callback( $example ) {
 * 	// Maybe modify $example in some way
 * 	return $example;
 * }
 * add_filter( 'example_filter', 'example_callback' );
 * </code>
 *
 * Since WordPress 1.5.1, bound callbacks can take as many arguments as are
 * passed as parameters in the corresponding apply_filters() call. The $accepted_args
 * parameter allows for calling functions only when the number of args match.
 *
 * <strong>Note:</strong> the function will return true whether or not the callback
 * is valid. It is up to you to take care. This is done for optimization purposes,
 * so everything is as quick as possible.
 *
 * @package WordPress
 * @subpackage Plugin
 *
 * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
 * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added, it doesn't need to run through that process.
 *
 * @since 0.71
 *
 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
 * @param callback $function_to_add The callback to be run when the filter is applied.
 * @param int      $priority        (optional) The order in which the functions associated with a particular action are executed. Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the action.
 *                                  Default 10.
 * @param int      $accepted_args   (optional) The number of arguments the function accepts.
 *                                  Default 1.
 * @return boolean true
 */
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)
{
    global $wp_filter, $merged_filters;
    $idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
    $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
    unset($merged_filters[$tag]);
    return true;
}