If you have a closure (or other callable) stored in an object property and you want to call it, you can use parentheses to disambiguate between it and a method call:
<?php
class Test
{
public $callable;
function __construct()
{
$this->callable = function($a) { return $a + 2; };
}
}
$t = new Test;
echo ($t->callable)(40);
?>