add_filter()添加过滤器

<?php add_filter( $tag, $function_to_add, $priority, $accepted_args );?>

/**
 * 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;
}

function test_func( $text, $var1, $var2 ){
    return $text . $var1 . $var2;
}
add_filter( 'test', 'test_func', 10, 3 );
echo apply_filters( 'test', '参数2', '参数3', '参数4' );