How a deprecated function can crash WordPress site while upgrading

Where…

The following files hold the deprecated functions:

• ~/wp-includes/deprectated.php
• ~/wp-includes/pluggable-deprectated.php
• …

Take a look at them and then through their replacement functions, where the calls to _deprecated_function() happen.

Why…

Normally a crash shouldn’t happen. In the “real” PHP world, you’d get an E_DEPRECATED Warning. In WordPress you’ll get a trigger_error() return caused by _deprecated_function() calls. The second trigger_error() argument isn’t used, so it defaults to E_USER_NOTICE and won’t stop your application from running.

How to turn it off…

You can turn this off either by using define( 'WP_DEBUG', false ); or by using a MU-Plugin that sets the filter to false:

<?php
defined('ABSPATH') or exit;
/** Plugin Name: (#105686) Turn off deprecated Warnings */
add_filter( 'deprecated_function_trigger_error', '__return_false' );