Wordpress 6.x / PHP 8.x deprecated warnings in development environment

Question

The problem:

I want to run Wordpress 6.x with PHP 8.x in development mode - meaning define('WP_DEBUG', true); but Wordpress 6.x partial support for PHP 8.x throws a lot of deprecated warnings which do a lot of mess on the screen and also mess with cookies and REST API.

Setting error_reporting(E_ALL & ~E_DEPRECATED & ~E_USER_DEPRECATED); in php.ini or wp-config.php does not solve the problem because Wordpress overwrites this setting when you set define('WP_DEBUG', true); which you want normally have for the development.

The solution is no straight forward and I couldn't find fast, either on Stack or anywhere else so below I will answer my own question and solve the problem for others.

Answer 1

change the line that sets error_reporting in wp-includes/load.php, probably around line 450:

if ( WP_DEBUG ) {
    error_reporting( E_ALL);

just add the & ~E_DEPRECATED bit so it becomes:

if ( WP_DEBUG ) {
    error_reporting( E_ALL & ~E_DEPRECATED );

[wordpress rant] - why would they not put something like this in the config file? In my (painful) experience of writing plugins & child themes for wordpress it's impossible to use debug mode not because of problems in my code but deprecated code in literally dozens of files throughout the codebase, ALWAYS. This renders WP_DEBUG useless. And don't get me started about how simple it would be to implement by default the multi-environment support I always have to hack into the wp-config.php file (I assume everyone else probably does this too) :-( [rant over(!!)]

Answer 2

The key to the solution is enable_wp_debug_mode_checks filter but to use it you have to do something special.

As the documentation says:

This filter runs before it can be used by plugins. It is designed for non-web runtimes. Returning false causes the WP_DEBUG and related constants to not be checked and the default PHP values for errors will be used unless you take care to update them yourself.

To use this filter you must define a $wp_filter global before WordPress loads, usually in wp-config.php.

Then when you disable this default error reporting overwrite - you can set your own. So the final solution looks like this:

define('WP_DEBUG', true);

if (WP_DEBUG) {
  define('WP_DEBUG_LOG', true);
  define('WP_DEBUG_DISPLAY', true);

  // disable friendly non informative error messages
  define('WP_DISABLE_FATAL_ERROR_HANDLER', true);

  $GLOBALS['wp_filter'] = [
    'enable_wp_debug_mode_checks' => [
      10 => [[
        'accepted_args' => 0,
        'function'      => function() { return false; },
      ]],
    ],
  ];
}

error_reporting(E_ALL & ~E_DEPRECATED & ~E_USER_DEPRECATED);

Unfortunately it turns off automatically also deprecated messages from your own code, but what can you do until Wordpress solves its deprecated code fragments.