How to know what functions are hooked to an action/filter?

Question

Is there a way to know what functions are hooked to a particular hook? For example if I'd like to know what functions are hooked to the wp_head hook.

Answer 1

Look into the global variable $wp_filter. See my plugin for a list of all comment filters for an example:

<?php
/*
Plugin Name: List Comment Filters
Description: List all comment filters on wp_footer
Version:     1.1
Author:      Fuxia Scholz
License:     GPL v2
*/

add_action( 'wp_footer', 'list_comment_filters' );

function list_comment_filters()
{
    global $wp_filter;

    $comment_filters = array ();
    $h1  = '<h1>Current Comment Filters</h1>';
    $out = '';
    $toc = '<ul>';

    foreach ( $wp_filter as $key => $val )
    {
        if ( FALSE !== strpos( $key, 'comment' ) )
        {
            $comment_filters[$key][] = var_export( $val, TRUE );
        }
    }

    foreach ( $comment_filters as $name => $arr_vals )
    {
        $out .= "<h2 id=$name>$name</h2><pre>" . implode( "\n\n", $arr_vals ) . '</pre>';
        $toc .= "<li><a href='#$name'>$name</a></li>";
    }

    print "$h1$toc</ul>$out";
}

Sample output for pre_comment_author_email:

array (
  10 => 
  array (
    'trim' => 
    array (
      'function' => 'trim',
      'accepted_args' => 1,
    ),
    'sanitize_email' => 
    array (
      'function' => 'sanitize_email',
      'accepted_args' => 1,
    ),
    'wp_filter_kses' => 
    array (
      'function' => 'wp_filter_kses',
      'accepted_args' => 1,
    ),
  ),
)

Answer 2

to see list of functions or actions hooked to a particular action hook you can use the following code.

global $wp_filter;
echo '<pre>';
var_dump( $wp_filter['wp_head'] );
echo '</pre>';

Answer 3

For debug-purposes a simple

global $wp_filter;
echo "<pre>" . print_r($wp_filter, true) . "</pre>";

would do it ...

Answer 4

This shows a more readable list of filters

function print_filters_for( $hook = '' ) {
    global $wp_filter;
    if( empty( $hook ) || !isset( $wp_filter[$hook] ) ) return;

    $ret='';
    foreach($wp_filter[$hook] as $priority => $realhook){
        foreach($realhook as $hook_k => $hook_v){
            $hook_echo=(is_array($hook_v['function'])?get_class($hook_v['function'][0]).':'.$hook_v['function'][1]:$hook_v['function']);
            $ret.=  "\n$priority $hook_echo";
        }

    }
     return $ret;
}

Answer 5

I found the answer from @Simone G useful, but it didn't take into account the fact, that sometimes Closures can be hooked. Here's my more verbose (and ugly) version:

if( isset($wp_filter[$filterName]) ){
    foreach( $wp_filter[$filterName] as $priority => $hooks){
        foreach ($hooks as $hook_k => $hook_v) {
            $hook_echo=(is_array($hook_v['function'])?get_class($hook_v['function'][0]).':'.$hook_v['function'][1]:$hook_v['function']);
            if(is_object($hook_echo) && ($hook_echo instanceof Closure)){
                $hook_echo="closure";
            }
            error_log($filterName." HOOKED (".serialize($priority)."): ".serialize($hook_k)."".serialize($hook_echo));
        }
    }
} else {
    error_log($filterName." NO FILTERS HOOKED");
}