php递归函数细节

<?php
/**
  *php递归函数细节
  *从1到5的阶乘
* */
header("Content-Type:text/html;charset=utf-8");

function sum($num){
     $tot = $num;
     echo "进入第{$num}层函数<hr style='border-style: dotted' />";
     if($num > 1){
             $tag = true;
            $sum = sum(--$num);  /*$sum = sum(--$num)之前是进入函数，之后是函数退出*/
             echo "<span style='color: red'>($tot => {$tot}) x ($sum => {$sum}) <b>=</b> </span>";
            $tot *= $sum;
      }
      
      echo "<span style='color: red'>($tot => {$tot})</span><br />";
      if($tag){
             ++$num;
             echo "退出第{$num}层函数<hr />";
      }else{
             echo "退出第{$num}层函数<hr />";
      }

      return $tot;
}
echo "等于：" . sum(5);

以上代码执行后的效果如下：