Description:
------------
Of the list of available type declarations[1], callable is unique in that class properties cannot be declared as such.

"Closure" can be used, but of course that prevents callables of other types (strings and arrays) from being stored.

This may be a WONTFIX and a corresponding comment in the documentation saying "you can't do that", but since the more general "callable" type declaration exists and can and is used elsewhere, there seems a priori no reason why it can't be used as a property.

[1] https://www.php.net/manual/en/functions.arguments.php#functions.arguments.type-declaration

Test script:
---------------
<?php
class Testing
{
	private callable $foo;

	public function __construct($foo = null)
	{
		$this->foo = $foo;
	}
}

new Testing(fn($a) => $a);


Expected result:
----------------
Nothing; script tests compilation only.


Actual result:
--------------
PHP Fatal error:  Property Testing::$foo cannot have type callable in C:\tmp\test.php on line 4