PHP Master | Understanding Recursion

<?php
function factorial ($number, $factorial = 1) {
    if ($number < 0) {
        throw new InvalidArgumentException('Number cannot be less than zero (0)');
    }
    if ($number == 0) {
        return $factorial;
    }
    else {
        return factorial($number - 1, $factorial * $number);
    }
}

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