POST variable verification

我想建立一個(gè)簡(jiǎn)單的折扣系統(tǒng)。我的想法是這樣的：用戶將輸入代碼&我想用PHP驗(yàn)證它并更新發(fā)票上的價(jià)格（可以做這部分，這就是為什么我只共享這部分代碼）

我希望看到你的解決方案，因?yàn)榈侥壳盀橹梗@根本不起作用，沒有回應(yīng)。

我的實(shí)際代碼（HTML表單）：

      <form method = "post" action = "<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
     <table>
        <tr>
           <td>code:</td>
           <td><input type = "text" name = "code">
           <span class = "error"><?php echo $codeErr;?></span>
           </td>
        </tr>
            
        <td>
           <input type = "submit" name = "submit" value = "Submit"> 
        </td>
            
     </table>
        
  </form>

PHP code:

      <?php
  
  
  $codeErr = "";
  $code = "";
     
     if ($_SERVER["REQUEST_METHOD"] == "POST") {
         
         
         
         
        if (empty($_POST["code"])) {
           $codeErr = "Code can not be blank.";
        }else {
           $code = test_input($_POST["code"]);
        }
     }
     
     function test_input($data) {
        $data = trim($data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        return $data;
        
        if ($code == "FIVE" ) {
           $codeErr = "OK";
        }else {
           $codeErr = "wrong code";
        }
     }
     
  ?>