WordPress plugin providing some functions to debug actions and filters.

debug-hooks.php

<?php

/**
 * Plugin Name: Debug Hooks
 * Author:      J.D. Grimes & Some other guy on the net
 * Version:     $ver$
 * Description: Adds some functions you can use to debug actions and filters.
 */

/**
 * List hooks.
 *
 * @since $ver$
 *
 * @param string $filter The hook to list. If omitted, all hooks will be listed.
 */
function list_hooks( $filter = false ) {

	global $wp_filter;

	$hooks = $wp_filter;
	ksort( $hooks );

	foreach( $hooks as $tag => $hook ) {

	    if ( false === $filter || $tag === $filter ) {
			dump_hook( $tag, $hook );
		}
	}
}

/**
 * Log hooks.
 *
 * @since $ver$
 *
 * @param string $filter The hook to log. If omitted, all hooks will be logged.
 */
function log_hooks( $filter = false ) {

	global $wp_filter;

	$hooks = $wp_filter;
	ksort( $hooks );

	foreach( $hooks as $tag => $hook ) {

	    if ( false === $filter || $tag === $filter ) {
			log_hook( $tag, $hook );
		}
	}
}

/**
 * List hook details live.
 *
 * @since $ver$
 *
 * @param string $hook The hook to list. If false, all hooks will be listed (default).
 */
function list_live_hooks( $hook = false ) {

    if ( false === $hook )
		$hook = 'all';

    add_action( $hook, 'list_hook_details', -1 );
}

/**
 * Log hook details live.
 *
 * @since $ver$
 *
 * @param string $hook The hook to log. If false, all hooks will be logged (default).
 */
function log_live_hooks( $hook = false ) {

    if ( false === $hook )
		$hook = 'all';

    add_action( $hook, 'log_hook_details', -1 );
}


/**
 * List the details about a hook.
 *
 * @since $ver$
 *
 * @param mixed $input The input passed to this hook. Passed through and ignored.
 */
function list_hook_details( $input = null ) {

    global $wp_filter;

    $tag = current_filter();

    if ( isset( $wp_filter[ $tag ] ) )
		dump_hook( $tag, $wp_filter[ $tag ] );

	return $input;
}

/**
 * Log the details about a hook.
 *
 * @since $ver$
 *
 * @param mixed $input The input passed to this hook. Passed through and ignored.
 */
function log_hook_details( $input = null ) {

    global $wp_filter;

    $tag = current_filter();

    if ( isset( $wp_filter[ $tag ] ) )
		log_hook( $tag, $wp_filter[ $tag ] );

	return $input;
}

/**
 * Get the details about a hook.
 *
 * @since $ver$
 */
function get_hook_details( $tag, $hook ) {

	ksort( $hook );

	$details = "Tag: {$tag}" . PHP_EOL;

	foreach( $hook as $priority => $functions ) {

		$did_priority = false;

		foreach( $functions as $function ) {

			if ( $function['function'] != 'list_hook_details' ) {

				if ( ! $did_priority ) {

					$details .= "Priority: {$priority}" . PHP_EOL;

					$did_priority = true;
				}

				$details .= "\t";

				if ( is_string( $function['function'] ) ) {

					$details .= $function['function'];

				} elseif ( $function['function'] instanceof Closure ) {

					$details .= 'Closure';

				} elseif ( is_string( $function['function'][0] ) ) {

					$details .= $function['function'][0] . ' -> ' . $function['function'][1];

				} elseif ( is_object( $function['function'][0] ) ) {

					$details .= "(object) " . get_class( $function['function'][0] ) . ' -> ' . $function['function'][1];

				} else {

					$details .= print_r( $function, true );
				}

				$details .= ' (' . $function['accepted_args'] . ')' . PHP_EOL;
			}
		}
    }

    return $details;
}

/**
 * Log the details about a hook.
 *
 * @since $ver$
 */
function log_hook( $tag, $hook ) {

	error_log( get_hook_details( $tag, $hook ) );
}

/**
 * Output a dump of information about a hook.
 *
 * @since $ver$
 *
 * @param unknown $tag
 * @param uknonwn $hook
 */
function dump_hook( $tag, $hook ) {

    echo '<pre>' . get_hook_details( $tag, $hook ) . '</pre>';
}

/**
 * Add a function to an action only once.
 *
 * @since $ver$
 *
 * @param string $action The name of the action hook.
 * @param string $function The name of the function to hook to the action.
 * @param int    $priority The priority this function should have in the hook stack.
 * @param int    $args     The number of arguments to pass to the function.
 */
function add_action_once( $action, $function, $priority, $args = 1 ) {

	add_action( $action, $function, $prioriy, $args );
	add_action(
		$action
		, function() use ( $action, $function, $priority, $args ) {

			remove_action( $action, $function, $priority, $args );
			remove_action( $action, __FUNCTION__, $priority, $args );
		}
		, $priority + 1
	);
}

/**
 * Add a function to a filter only once.
 *
 * @since $ver$
 *
 * @param string $filter   The name of the action hook.
 * @param string $function The name of the function to hook to the filter.
 * @param int    $priority The priority this function should have in the hook stack.
 * @param int    $args     The number of arguments to pass to the function.
 */
function add_filter_once( $filter, $function, $priority = 10, $args = 1 ) {

	add_filter( $filter, $function, $priority, $args );
	add_filter(
		$filter
		, function( $filtered ) use ( $filter, $function, $priority, $args ) {

			remove_filter( $filter, $function, $priority, $args );
			remove_filter( $filter, __FUNCTION__, $priority, $args );

			return $filtered;
		}
		, $priority + 1
	);
}

typo on line 215 for $priority