y.php

<?php

$factorial = call_user_func(
    function ($le) {
        return call_user_func(
            function ($f) {
                return $f($f);
            },
            function ($f) use ($le) {
                return $le(function ($x) use ($f) {
                    return call_user_func($f($f), $x);
                });
            }
        );
    },
    function ($factorial) {
        return function ($x) use ($factorial) {
            if ($x > 0) {
                return $x * $factorial($x - 1);
            }
            return 1;
        };
    }
);

var_dump($factorial(1));
var_dump($factorial(2));
var_dump($factorial(3));
var_dump($factorial(4));

I suppose I could use the y combinator, but this is simpler:

<?php

$factorial = function ($x) use (&$factorial) {
    if ($x > 0) {
        return $x * $factorial($x - 1);
    }
    return 1;
};

:)

Who needs recursion for a factorial though:

function factorial($number)
{
    $factorial = 1;
    while ($number > 1) {
        $factorial *= $number--;
    }
    return $factorial ;
}

@MarkBaker we can find an iterative solution to almost any problem that we can solve with recursion. The point was to illustrate that it is possible to make a closure recursive in PHP. Which is not a straight forward conclusion, as there is no keyword to refer to the closure itself from within the closure.