wp_fatal_error_handler_enabled doesn't actually do anything

[programmin]

Suppose you want to turn off the error handling... Try this in a mu-plugin:

<?php

function somethingg() { echo 'my code runs'; return false;
}
somethingg();
add_filter( 'wp_fatal_error_handler_enabled', 'somethingg' );

And add this to error-protection.php WP:

<?php
function wp_register_fatal_error_handler() {
        if ( ! wp_is_fatal_error_handler_enabled() ) {
                return;
        }
echo 'FATAL HANDLER = TRUE';

Now if you get an error (eg new NoneClass()) you will get the output:

FATAL HANDLER = TRUEmy code runs

There has been a critical error on your website. Please check your site admin email inbox for instructions.

So the filter apparently never is seen because the handler runs before the add_filter even had a chance to set it.

[johnbillion]

I believe the intention of this filter is that it can only be used before WordPress initialises, for example by constructing the $wp_filter array inside wp-config.php. @schlessera might be able to confirm this.

[SergeyBiryukov]

Replying to johnbillion:

I believe the intention of this filter is that it can only be used before WordPress initialises, for example by constructing the $wp_filter array inside wp-config.php.

That's my understanding too, as it runs too early to be used in a "normal" way. If that's the case, it could use a better documentation.

[programmin]

add_filter only works if WP is up? So yes if there is a specific way to do this then documentation should really show that the filter doesn't actually work, doesn't work like all the others work.

[johnbillion]

In 49489:

Docs: Add more information about how to use filters that run before WordPress initialises.

Fixes #50134