PHP: ArgumentCountError on method with optional parameter

Question

PHP 8.1. I declared a method with this signature:

public static function if(mixed $condition, mixed $subject = null, \Closure $callback)

And called it with this call:

CLASS::if(condition: is_numeric(...), callback: do_something(...));

Yet I still got this exception:

ArgumentCountError: CLASS::if(): Argument #2 ($subject) not passed

Why? $subject clearly is set to default to null.

Answer 1

Since the $callback parameter has no default value, it's a required argument, and so is every argument before it. Therefore, $subject = null doesn't mean null is a default value; it only means $subject is nullable (everything other than null would have triggered a deprecation notice).

To solve the problem, depending on the behaviour you're expecting, you can either place the $subject parameter at the end, or add a default value to the $callback parameter.