[wp-trac] [WordPress Trac] #60235: call_user_func_array invalid callback error severity change in PHP 8

[WordPress Trac]

#60235: call_user_func_array invalid callback error severity change in PHP 8
-------------------------+-----------------------------
 Reporter:  david.binda  |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  General      |    Version:
 Severity:  normal       |   Keywords:
  Focuses:               |
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 While testing the PHP 8.x on WordPress.com, PHP fatal errors for invalid
 callbacks to filters and actions were noticed.

 When running WordPress on the PHP 7.x, an invalid callback to a
 filter/action produces a warning and thus does not stop the script
 execution. Upgrading to PHP 8.x leads to a fatal error in the same
 situation.

 I wonder whether there should be a `is_callable` check in the
 `WP_Hook::apply_filters` preventing fatal error from happening (plus some
 `_doing_it_wrong` notice), as it would make the transition from PHP 7.x to
 8.x easier in cases where the error severity change applies.

 To replicate the issue should be as easy as adding following to a mu-
 plugin:

 {{{#!php
 <?php
 add_action( 'init', 'non_existing_function' );
 }}}

 Produces warning in PHP 7.x:

 `Warning: call_user_func_array() expects parameter 1 to be a valid
 callback, function 'non_existing_function' not found or invalid function
 name in wp-includes/class-wp-hook.php on line 324`

 and fatal error in PHP 8.x:

 `Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1
 ($callback) must be a valid callback, function "non_existing_function" not
 found or invalid function name in wp-includes/class-wp-hook.php:324`

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Ticket URL: <https://core.trac.wordpress.org/ticket/60235>
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