php7-factorial.php

function factorial($n)
{
    if ($n == 1) {
        return $n;
    }
    return $n * factorial($n-1);
}

function tail_factorial($n, $acc = 1)
{
    if ($n == 1) {
        return $acc;
    }
    return tail_factorial($n-1, $n * $acc);
}

function unrolled_factorial($n)
{
    $acc = 1;
    while ($n > 1) {
        $acc *= $n--;
    }
    return $acc;
}

function goto_factorial($n)
{
    $acc = 1;
f:
    if ($n == 1) {
        return $acc;
    }
    $acc *= $n--;
    goto f;
}

function trampo_factorial($n, $acc = 1)
{
    if ($n == 1) {
        return $acc;
    }
    return function() use ($n, $acc) { return trampo_factorial($n-1, $n * $acc); };
}

function trampoline(callable $c, ...$args)
{
    while (is_callable($c)) { $c = $c(...$args); } return $c;
}

echo trampoline('trampo_factorial', 42);