Ajax 调用 PHP 文件，但某些数据不返回 [closed]

问：

返回“undone”或“error”。但是“完成”不会返回。 当邮件地址存在时，“撤消”返回，将新用户插入数据库有效，但“完成”不会返回。我希望在插入用户时看到 $data[“status”] 中的“undone”返回。我该如何解决？

js

$(document).ready(function(){
   $("#form").submit(function(){
     $.ajax({
       url: "file.php",
       type: "POST",
       dataType: "json",
       data: $('#form').serialize(),
       success: function(data){
         alert(data.status);
       }
     })
   })
 })

file.php

<?php
   session_start();
   if (isset($_POST['submit'])){

     include 'config.php';
     $data = array();
     $name = $conn -> real_escape_string($_POST['username']);
     $mail = $conn -> real_escape_string($_POST['mail']);
     $psw = $conn -> real_escape_string($_POST['password']);
     $code = $conn -> real_escape_string(uniqid());
 
 
 
     $qry = $conn->query("SELECT * FROM member WHERE mail='{$mail}'");
     if (mysqli_num_rows($qry)>0) {
       $data["status"] = "undone";
     }else {
       $qry2 = $conn->prepare("INSERT INTO member (name, mail, psw, code) VALUES ('{$name}','{$mail}','{$psw}','{$code}')");
       if($qry2){
         $qry2->execute();
         $_SESSION['SESSION_MAILADDRESS'] = $mail;
         $data["status"] = "done";
 
         mysqli_free_result($qry2);
       }else {
         $data["status"] = "error";
       }
     }
     echo json_encode($data);
     mysqli_free_result($qry);
     $conn->close();
   }
  ?>