Check the hook callback parameter count ?

[jrfnl]

What problem would the enhancement address for VIP?

Prevent potential fatal ArgumentCount errors.

Describe the solution you'd like

Inspired by the AlwaysReturnInFilter sniff, I was thinking that once the abstract hook/callback sniff exists, it wouldn't be that hard to create a sniff which would check synchronicity between the parameters requested from a hook-in and the callback function declaration.

This would apply to both add_filter() as well as add_action().

Such a sniff would also be a candidate for (eventually moving to) WPCS, but I wanted to discuss the general idea here first, largely to see if the potential issue described above actually occurs in real life situations.

What code should be reported as a violation?

// Error: Two parameters expected, one (implicit) requested.
add_filter( 'the_title', function( $title, $id ) {
	// Do something.
	return $title;
} );

// Warning: One parameter expected, two requested.
add_filter( 'the_title', function( $title ) {
	// Do something.
	return $title;
}, 10, 2 );

What code should not be reported as a violation?

// OK: Two parameters expected, two requested.
add_filter( 'the_title', function( $title, $id ) {
	// Do something.
	return $title;
}, 10, 2 );

// OK: Three parameters expected, two requested, but third parameter is optional,
// which can be done by design to allow direct calls to the function to pass it.
function prefix_title_filter( $title, $id, $optional = null ) {
	// Do something.
	return $title;
}
add_filter( 'the_title', 'prefix_title_filter', 10, 2 );

Additional context

Future scope

• Once a list of WP native hooks + the amount of parameters they pass is known (via a trait in WPCS for instance), an additional check could be added that the $accepted_args parameter value is never higher than the number of passed parameters when the hook is called.

function prefix_title_filter( $title, $id, $optional = null ) {
	// Do something.
	return $title;
}

// The `the_title` filter only passes two arguments max, so the code here is an error.
add_filter( 'the_title', 'prefix_title_filter', 10, 3 );

• For PHP native functions used in callbacks, reflection could be used to see how many (required) parameters these take and match that with the $accepted_args parameter.
• For WP native functions a list based solution would be needed to do the same.

Relevant links

• https://developer.wordpress.org/reference/functions/add_action/
• https://developer.wordpress.org/reference/functions/add_filter/