A must-use plugin for WordPress developers figuring out what filter callback changes a value passed to the `apply_filters` function

apply-filters.php

<?php
/*
	Plugin Name: Apply Filters debug
	Description: A debug function for figuring out what filter's callback changed the value
	Version: 0.1
	Author: Automattic, david-binda
	Author URI: http://www.automattic.com/
*/

/*  This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

/*
  # Installation
  
  Copy this file to your WordPress installation's mu-plugins directory (located in wp-content/mu-plugins)
  
  # Usage
  
  Call the `swpd_apply_filters_debug` function right before the code which is calling the `apply_filters`.
  It collects the data and outputs the logging to PHP Error log or directly to stdout in case
  it's used via WP CLI - eg. in wp shell

  Example of figuring out what callback changes the scheme of a home URL:

  ```
  $ wp shell
  wp> swpd_apply_filters_debug( 'set_url_scheme', true );
  wp> home_url();
  ```

  Example logging output:

  ```
  [24-Mar-2017 12:33:50 UTC] Initial value: 'https://vip.wordpress.com'
  [24-Mar-2017 12:33:50 UTC] array (
    'value' => '\'http://vip.wordpress.com\'',
    'idx' => '\'wpcom_only_use_https_with_valid_ssl_cert\'',
    'the_' => 'array (
      \'function\' => \'wpcom_only_use_https_with_valid_ssl_cert\',
      \'accepted_args\' => 3,
    )',
    'priority' => 10,
  )
  ```
*/

/**
 * @param $filter_to_debug What filter is being debugged
 * @param bool|false $only_changed Whether to log only filters which change the value or all. Defaults to all.
 *
 * @return void
 */
function swpd_apply_filters_debug( $filter_to_debug, $only_changed = false ) {
	new SWPD_apply_filters( $filter_to_debug, $only_changed );
}

class SWPD_apply_filters {

	private $filter_to_debug = null;

	private $only_changed = false;

	private $swpd_filter_prev_value = null;

	public function __construct( $filter, $only_changed = false ) {
		$this->filter_to_debug = $filter;
		$this->only_changed = $only_changed;
		add_filter( 'all', array( $this, 'filter_all' ), 10, 2 );
	}

	public function filter_all( $action, $value = null ) {
		if ( current_filter() === $this->filter_to_debug ) {
			$this->add_debugging( $value );
		}
		return $value;
	}

	public function add_debugging( $value ) {
		global $wp_filter;
		$this->log( 'Initial value:', $value );
		$this->swpd_filter_prev_value = $value;
		if ( true === array_key_exists( $this->filter_to_debug, $wp_filter ) ) {
			$filters = $wp_filter[$this->filter_to_debug]->callbacks;
			$wp_filter[$this->filter_to_debug]->callbacks = array();
			$i = 0;
			foreach( $filters as $priority => $thes_ ) {
				foreach( $thes_ as $idx => $the_ ) {
					$wp_filter[$this->filter_to_debug]->callbacks[$priority][$idx] = $the_;
					$wp_filter[$this->filter_to_debug]->callbacks[$priority][ _wp_filter_build_unique_id( $this->filter_to_debug, array( $this, 'debug' ), $priority ) . $i++ ] = array(
						'function' => function( $value ) use ( $idx, $priority, $the_ ) { $this->debug( $value, $idx, $the_, $priority ); return $value; },
						'accepted_args' => 1,
					);
				}
			}
		}
	}

	public function debug( $value, $idx, $the_, $priority ) {
		if ( false === $this->only_changed || $this->swpd_filter_prev_value !== $value ) {
			$this->log( '', array(
				'value' => $value,
				'idx' => $idx,
				'the_' => $the_,
				'defined_in' => $this->defined_in( $the_ ),
				'priority' => $priority
			) );
		}
		$this->swpd_filter_prev_value = $value;
		return $value;
	}

	public function defined_in( $the_ ) {
		$return = 'N/A';
		// Handle function and closures.
		if ( true === is_string( $the_['function'] ) || true === is_a( $the_['function'], '\Closure' ) ) {
			$the_reflection = new ReflectionFunction( $the_['function'] );
			$return = $the_reflection->getFileName() . '#L' . $the_reflection->getStartLine();
		}
		// Handle class' methods
		if ( true === is_array( $the_['function'] ) ) {
			$the_reflection = new ReflectionMethod( $the_['function'][0], $the_['function'][1] );
			$return = $the_reflection->getFileName() . '#L' . $the_reflection->getStartLine();
		}
		return $return;
	}

	public function log( $message, $data ) {
		error_log( $message . ' ' . var_export( $data, true ) );
		if ( true === defined( 'WP_CLI' ) && true === WP_CLI ) {
			WP_CLI::Line( $message . ' ' . var_export( $data, true ) );
		}
	}
}