tailrecursion.php

<?php
class TailRecursion
{
    public $func;
    public $acc;
    public $recursing;

    public function tail()
    {
        return call_user_func_array($this->func, func_get_args());
    }

    public function getClosure($fn)
    {
        $this->acc = array();
        $this->recursing = false;
        $fn = $fn->bindTo($this);

        return $this->func = function() use ($fn) {

            $this->acc[] = func_get_args();

            if ( ! $this->recursing) {
                $this->recursing = true;

                while ($this->acc) {
                    $ret = call_user_func_array($fn, array_shift($this->acc));
                }

                $this->recursing = false;

                return $ret;
            }
        };
    }
}

function tailrec($func)
{
    $tail = new TailRecursion();
    return $tail->getClosure($func);
}

$fac = tailrec(function($n, $acc = 1) {
    if ($n == 1) {
        return $acc;
    }

    return $this->tail($n - 1, $acc * $n);
});

echo $fac(4); // 1 * 2 * 3 * 4 = 24