Introduce is_callable check into apply_filters function to avoid failure of call_user_func_array causing all filters to fail.

[garrett-eclipse]

Hello,

Just wanted to suggest an improvement for the apply_filters function to avoid failures introduced by custom code or plugins that invalidly set an add_filter without providing the scope or misspelling the function.

Currently if a filter is applied incorrectly... in the case I came across I had a custom plugin which I missed wrapping the array($this,) around the function name. This caused an error in debug for call_user_func_array expecting valid callback. Due to this NULL is returned and all other filters are basically ignored.

Would be nice to do an is_callable check before execution of the call_user_func_array so as to skip any broken add_filters applied. I'd say still log an error so people can troubleshoot and find out why their filter failed, but would be nicer than returning null.

Thank you

[SergeyBiryukov]

#49002 was marked as a duplicate.

[sjoerdlinders]

Replying to SergeyBiryukov:

#49002 was marked as a duplicate.

With the solution to solve the problem in file: class-wp-hooks.php

public function apply_filters( $value, $args ) {
    ...
    foreach ( $this->callbacks[ $priority ] as $the_ ) {
        if( ! $this->doing_action ) {
            $args[ 0 ] = $value;
        }

        // Check if callable function or method exists.
        if ( !isset( $the_['function'] ) || !is_callable( $the_['function'] ) ) continue;

        ....
    }

[johnbillion]

Silently discarding an invalid callback isn't a good idea because it turns a visible error into an invisible one.

PHP errors exist for a reason and I think this is a valid case for an error. It's unfortunate that subsequent callbacks receive a null value, so if this guard condition is going to be put into place it needs to trigger a PHP error too.

This will need performance profiling too to ensure there's minimal performance impact.

[SergeyBiryukov]

Related: #51894

Add check for the callback function value passed to apply_filters, to determine if the value is empty or not callable. Trigger a _doing_it_wrong() call if an invalid value is passed to make sure the error is not silenced.

Trac ticket: https://core.trac.wordpress.org/ticket/38116

[johnbillion]

#60235 was marked as a duplicate.

[johnbillion]

I'm going to make an executive decision and close this for the same reason as #51894. An invalid callback is a clear developer error and should not be ignored. The fact that what used to be a PHP warning is now a fatal error is unfortunate but valid, and it does make it much more visible that a problem has occurred.