Let us further understand the if statement of PHP flow control statement! ! ! (Source code attached)

<form action=&#39;&#39; method=&#39;get&#39;>
     <input type="text" name="num1">
     <select name="fh">
        <option value="jia">+</option>
        <option value="jian">-</option>
        <option value="c">*</option>
        <option value="chu">/</option>
        <option value="qy">%</option>
      </select>
      <input type="text" name="num2">
      <input type="submit" value="运算"/> 
</form>

<?php
     $num1 = $_GET[&#39;num1&#39;];
     $num2 = $_GET[&#39;num2&#39;];
     $fh =$_GET[&#39;fh&#39;];

     if(!is_numeric($num1) || !is_numeric
          ($num2)){
              echo &#39;请输入数值类型&#39;;
          }
          if($fh==&#39;jia&#39;){
              echo $num1 . &#39;+&#39; . $num2 . &#39;=&#39; .
              ($num1+$num2);
          }
          if($fh==&#39;jian&#39;){
            echo $num1 . &#39;-&#39; . $num2 . &#39;=&#39; .
            ($num1-$num2);
        }
        if($fh==&#39;c&#39;){
            echo $num1 . &#39;*&#39; . $num2 . &#39;=&#39; .
            ($num1*$num2);
        }
        if($fh==&#39;chu&#39;){
            echo $num1 . &#39;/&#39; . $num2 . &#39;=&#39; .
            ($num1/$num2);
        }
        if($fh==&#39;qy&#39;){
            echo $num1 . &#39;%&#39; . $num2 . &#39;=&#39; .
            ($num1%$num2);
        }




?>