Calculates the factorial of a number.

Use recursion.
If $n is less then or equal to 1, return 1.
Otherwise, return the product of $n and the factorial of $n -1.

代码实现

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

  return $n * factorial($n - 1);
}

使用样例

factorial(6); // 720