Note that when you use implementation of factorial that ClaudiuS made, you get results even if you try to calculate factorial of number that you normally can't, e.g. 2.5, -2, etc. Here is safer implementation:
<?php
/**
 * Calculates a factorial of given number.
 * @param string|int $num
 * @throws InvalidArgumentException
 * @return string
 */
function bcfact($num)
{
    if (!filter_var($num, FILTER_VALIDATE_INT) || $num <= 0) {
        throw new InvalidArgumentException(sprintf('Argument must be natural number, "%s" given.', $num));
    }

    for ($result = '1'; $num > 0; $num--) {
        $result = bcmul($result, $num);
    }

    return $result;
}
?>