WordPress: How do I get all the registered functions for 'the_content' filter

Simple function to print from the filter array?

function print_filters_for( $hook = '' ) {    global $wp_filter;    if( empty( $hook ) || !isset( $wp_filter[$hook] ) )        return;    print '<pre>';    print_r( $wp_filter[$hook] );    print '</pre>';}

Call it where you need it.

print_filters_for( 'the_content' );

This is a bit more advanced example, that will, in addition to data from $wp_filter array, show the path of the file where the hook is attached, as well as the line in code where the function is defined.

To get a basic list of functions hooked on a specific action ( or filter ) it is enough to fetch the items from the filter array, but since the functions can be attached in various ways ( as a class method or closure ) that list will contain a ton of irelevant data that includes objects presented as string. This example will display only the relevant data, in order of priority:

• function name ( depending on callbacks syntax ):
  • function callback: 'function_name'
  • object method: array( $object, 'function_name' )
  • static class method: array( 'class_name', 'function_name' ) and 'class_name::function_name'
  • closure: function() {}
  • relative static class method: array( 'class_name', 'parent::function_name' )
• accepted args
• file name
• start line
• id
• priority

function list_hooks( $hook = '' ) {    global $wp_filter;    if ( isset( $wp_filter[$hook]->callbacks ) ) {              array_walk( $wp_filter[$hook]->callbacks, function( $callbacks, $priority ) use ( &$hooks ) {                       foreach ( $callbacks as $id => $callback )                $hooks[] = array_merge( [ 'id' => $id, 'priority' => $priority ], $callback );        });             } else {        return [];    }    foreach( $hooks as &$item ) {        // skip if callback does not exist        if ( !is_callable( $item['function'] ) ) continue;        // function name as string or static class method eg. 'Foo::Bar'        if ( is_string( $item['function'] ) ) {            $ref = strpos( $item['function'], '::' ) ? new ReflectionClass( strstr( $item['function'], '::', true ) ) : new ReflectionFunction( $item['function'] );            $item['file'] = $ref->getFileName();            $item['line'] = get_class( $ref ) == 'ReflectionFunction'                 ? $ref->getStartLine()                 : $ref->getMethod( substr( $item['function'], strpos( $item['function'], '::' ) + 2 ) )->getStartLine();        // array( object, method ), array( string object, method ), array( string object, string 'parent::method' )        } elseif ( is_array( $item['function'] ) ) {            $ref = new ReflectionClass( $item['function'][0] );            // $item['function'][0] is a reference to existing object            $item['function'] = array(                is_object( $item['function'][0] ) ? get_class( $item['function'][0] ) : $item['function'][0],                $item['function'][1]            );            $item['file'] = $ref->getFileName();            $item['line'] = strpos( $item['function'][1], '::' )                ? $ref->getParentClass()->getMethod( substr( $item['function'][1], strpos( $item['function'][1], '::' ) + 2 ) )->getStartLine()                : $ref->getMethod( $item['function'][1] )->getStartLine();        // closures        } elseif ( is_callable( $item['function'] ) ) {                 $ref = new ReflectionFunction( $item['function'] );                     $item['function'] = get_class( $item['function'] );            $item['file'] = $ref->getFileName();            $item['line'] = $ref->getStartLine();        }           }    return $hooks;}

Since hooks can be added and removed throughout the entire runtime, the output depends on at what point the function is called ( wp_footer action is a good place to get the complete list )

print_r example for the_content filter:

Array(    [0] => Array        (            [id] => 000000004c8a4a660000000011808a14run_shortcode            [priority] => 8            [function] => Array                (                    [0] => WP_Embed                    [1] => run_shortcode                )            [accepted_args] => 1            [file] => C:\xampp\htdocs\wordpress\wp-includes\class-wp-embed.php            [line] => 58        )    [1] => Array        (            [id] => wptexturize            [priority] => 10            [function] => wptexturize            [accepted_args] => 1            [file] => C:\xampp\htdocs\wordpress\wp-includes\formatting.php            [line] => 41        )    [2] => Array        (            [id] => 0000000006c5dc6d0000000064b1bc8e            [priority] => 10            [function] => Closure            [accepted_args] => 1            [file] => C:\xampp\htdocs\wordpress\wp-content\plugins\plugin\plugin.php            [line] => 16        )    .....

Edit: 2017-05-05

• adapted for WP_Hook class
• added priority
• fixed: error raised if callback does not exists, although WordPress also raises a warning for that
• fixed: hook with the same id but different priority overwrites the previous one

Based on the same idea of print_filters_for, an extended function for those who need not only print but return as well:

function filters_for( $hook = '', $return = FALSE ) {    global $wp_filter;    if( empty( $hook ) || !isset( $wp_filter[$hook] ) )        return;    if( $return ) {        ob_start();    }    print '<pre>';    print_r( $wp_filter[$hook] );    print '</pre>';    if( $return ) {        return ob_get_flush();    }}

Output:

filters_for( 'the_content' );

Get and output:

$output = filters_for( 'the_content', true );echo $output;