php 递归法求n的阶乘

<?php  
 function f($n)
  {
      if($n <0)
      {
          echo "不是整数\n";
          exit;
      }
      else if($n == 0 || $n == 1)
      {
          return $n;
      }
      else
      {
        return f($n-1)*$n;
      }

  }

  echo f(6);
  ?>