The Little Schemer - Y combinator in PHP

YCombinator.php

<?php
/**
 * “The Little Schemer” - Y combinator in PHP
 * @author: Joris Garonian
 */

function Y($g) {
    return call_user_func(function ($f) {
        return $f($f);
    }, function ($f) use ($g) {
        return $g(function ($x) use ($f) {
            return call_user_func($f($f), $x);
        });
    });
}

$factorial = Y(function ($x) {
    return function ($n) use ($x) {
        return $n <= 2 ? $n : $n * $x($n - 1);
    };
});

echo $factorial(3);