It is said that deprecated functions used in plugins & themes can crash a wordress website while upgrading to newer version, I want to see an example of deprecated function that can cause a crash.

Can someone show me an example?

Many Thanks!

  • 1
    They'll trigger a fatal error if they were finally removed in the new version yet were still in use. But nothing different than a regular PHP "call to undefined function" (or WSOD if debugging is disabled). – TheDeadMedic Jul 8 '13 at 10:10
  • @TheDeadMedic Should be an answer :) – kaiser Jul 8 '13 at 10:48


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.


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:

defined('ABSPATH') or exit;
/** Plugin Name: (#105686) Turn off deprecated Warnings */
add_filter( 'deprecated_function_trigger_error', '__return_false' );
  • nice answer but I want to see an example of why WP sites crash while upgrading, can you please give me an example? – Tahir Yasin Jul 8 '13 at 10:46
  • @TahirYasin I have no example, as there is none. Deactivate debug or use the MU-Plugin and you're fine. As I explained, it's an E_NOTICE, not an E_ERROR. The only thing I could imagine is that it echos inside some call where nothing should echo. For example during plugin activation. Just update your question with a real world example. So far you're too abstract. – kaiser Jul 8 '13 at 10:48

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