R Debug (set of dump helpers for debug)

r-debug.php

<?php

/*
Plugin Name: R Debug
Description: Set of dump helpers for debug.
Author: Andrey "Rarst" Savchenko
Author URI: https://www.rarst.net/
License: MIT
 */

/**
 * Class with static dump methods
 */
class R_Debug {

	/**
	 * List basic performance stats
	 *
	 * @param bool $visible display or only include in source
	 */
	static function list_performance( $visible = false ) {

		if ( defined( 'DOING_AJAX' ) ) {
			return;
		}

		$stat = sprintf(
			'%d queries in %s seconds, using %.2fMB memory',
			get_num_queries(),
			timer_stop( 0, 3 ),
			memory_get_peak_usage() / 1024 / 1024
		);

		echo $visible ? $stat : "<!-- {$stat} -->";
	}

	/**
	 * List defined constants
	 *
	 * @param bool|string $filter limit to matching names or values
	 */
	static function list_constants( $filter = false ) {

		$constants = get_defined_constants();

		if ( false !== $filter ) {
			$temp = array();

			foreach ( $constants as $key => $constant ) {
				if ( false !== stripos( $key, $filter ) || false !== stripos( $constant, $filter ) ) {
					$temp[ $key ] = $constant;
				}
			}

			$constants = $temp;
		}

		ksort( $constants );
		self::dump( $constants );
	}

	/**
	 * Concatenate print_r of all input and echo in pre tags.
	 */
	static function dump() {

		$output = '';

		foreach ( func_get_args() as $arg ) {
			$output .= print_r( $arg, true );
		}

		echo '<pre>' . $output . '</pre>';
	}

	/**
	 * List cron entries with time remaining till next run
	 */
	static function list_cron() {

		$cron = _get_cron_array();

		echo '<pre>';

		$offset = get_option( 'gmt_offset' ) * 3600;

		foreach ( $cron as $time => $entry ) {

			$when = '<strong>In ' . human_time_diff( $time ) . '</strong> (' . $time . ' ' . date_i18n( DATE_RSS, $time + $offset ) . ')';
			echo "<br />&gt;&gt;&gt;&gt;&gt;\t{$when}<br />";

			foreach ( array_keys( $entry ) as $function ) {
				echo "\t{$function}<br />";
				self::list_hooks( $function );
			}
		}

		echo '</pre>';
	}

	/**
	 * List hooks as currently defined
	 *
	 * @param bool|string $filter limit to matching names
	 */
	static function list_hooks( $filter = false ) {

		global $wp_filter;

		$skip_filter = empty( $filter );
		$hooks       = $wp_filter;
		ksort( $hooks );

		foreach ( $hooks as $tag => $hook ) {
			if ( $skip_filter || false !== strpos( $tag, $filter ) ) {
				self::dump_hook( $tag, $hook );
			}
		}
	}

	/**
	 * Output hook info
	 *
	 * @param string        $tag  hook name
	 * @param WP_Hook|array $hook hook data
	 */
	static function dump_hook( $tag, $hook ) {

		if ( $hook instanceof WP_Hook ) {
			$hook = $hook->callbacks;
		}

		ksort( $hook );

		echo "<pre>&gt;&gt;&gt;&gt;&gt;\t<strong>{$tag}</strong><br />";

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

			foreach ( $functions as $function ) {
				echo "\t";

				$callback = $function['function'];

				if ( is_string( $callback ) ) {
					echo $callback;
				} elseif ( is_a( $callback, 'Closure' ) ) {
					$closure = new ReflectionFunction( $callback );
					echo 'closure from ' . $closure->getFileName() . '::' . $closure->getStartLine();
				} elseif ( is_object( $callback ) ) {
					$class = new ReflectionClass( $callback );
					$name  = $class->getName();
					if ( 0 === strpos( $name, 'class@anonymous' ) ) {
						echo 'anonymous class from ' . $class->getFileName() . '::' . $class->getStartLine();
					} else {
						echo $name;
					}
				} elseif ( is_string( $callback[0] ) ) { // static method call
					echo $callback[0] . '::' . $callback[1];
				} elseif ( is_object( $callback[0] ) ) {
					echo get_class( $callback[0] ) . '->' . $callback[1];
				}

				echo ( 1 == $function['accepted_args'] ) ? '<br />' : " ({$function['accepted_args']}) <br />";
			}
		}

		echo '</pre>';
	}

	/**
	 * Enable live listing of hooks as they run
	 *
	 * @param bool|string $hook limit to matching names
	 */
	static function list_live_hooks( $hook = false ) {

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

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

	/**
	 * Handler for live hooks output
	 *
	 * @param mixed $input
	 *
	 * @return mixed
	 */
	static function list_hook_details( $input = null ) {

		global $wp_filter;

		$tag = current_filter();

		if ( isset( $wp_filter[ $tag ] ) ) {
			self::dump_hook( $tag, $wp_filter[ $tag ] );
		}

		return $input;
	}

	/**
	 * List active plugins
	 */
	static function list_plugins() {

		self::dump( get_option( 'active_plugins' ) );
	}

	/**
	 * List post's fields, custom fields, and terms
	 *
	 * @param int $post_id
	 */
	static function list_post( $post_id = null ) {

		if ( empty( $post_id ) ) {
			$post_id = get_the_ID();
		}

		self::dump(
			get_post( $post_id ),
			get_post_custom( $post_id ),
			wp_get_post_terms( $post_id, get_post_taxonomies( $post_id ) )
		);
	}

	/**
	 * List performed MySQL queries
	 */
	static function list_queries() {

		global $wpdb;

		if ( ! defined( 'SAVEQUERIES' ) || ! SAVEQUERIES ) {

			trigger_error( 'SAVEQUERIES needs to be defined', E_USER_NOTICE );

			return;
		}

		echo '<pre>';

		foreach ( $wpdb->queries as $query ) {

			list( $request, $duration, $backtrace ) = $query;
			$duration  = sprintf( '%f', $duration );
			$backtrace = explode( ',', $backtrace );
			$backtrace = trim( array_pop( $backtrace ) );

			if ( 'get_option' == $backtrace ) {

				preg_match_all( '/\option_name.*?=.*?\'(.+?)\'/', $request, $matches );
				$backtrace .= "({$matches[1][0]})";
			}

			echo "<br /><code>{$request}</code><br />{$backtrace} in {$duration}s<br />";
		}

		echo '<br /></pre>';
	}

	/**
	 * Run EXPLAIN on provided MySQL query or last query performed.
	 *
	 * @param string $query
	 */
	static function explain_query( $query = '' ) {

		/** @var wpdb $wpdb */
		global $wpdb;

		if ( empty( $query ) ) {
			$query = $wpdb->last_query;
		}

		self::dump(
			$query,
			$wpdb->get_results( 'EXPLAIN EXTENDED ' . $query ),
			$wpdb->get_results( 'SHOW WARNINGS' )
		);
	}
}

Kudos for you guys, this helped me to debug hooks @Rarst & @kaorukobo