WordPress: 函數 add_action與add_filter的區別

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 add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1) 
{
 return add_filter($tag, $function_to_add, $priority, $accepted_args);
}