如何在检查电子邮件的网站的注册表单中使用Google ReCaptcha，用户是否已注册？

Question

我正在使用AJAX在模板上使用Google ReCaptacha为我的网站创建一个注册表单，其中从MYSQL数据库检查电子邮件是否已注册用户，我该怎么办？去做？ 请记住表单和ReCaptcha是Boootstrap模式。

这是我的Bootstrap模式表单。

<div class="modal" id="signup">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">x</button>
                <h4 class="modal-title text-center">Create an account</h4>
            </div>
            <div class="modal-body">
                <form action="process_registration.php" method="post">
                    <div class="form-group">
                        <div id="usernamemessage"></div>
                        <input type="text" name="username" class="btn-block input-sm" required placeholder="Write a Username for your account" id="username">
                    </div>
                    <div class="form-group">
                        <div id="emailmessage"></div>
                        <input type="email" name="email" class="btn-block input-sm" required placeholder="Enter Your Email" id="email">
                    </div>
                    <div class="form-group">
                        <input type="password" name="password" class="btn-block input-sm" required placeholder="Enter  a Strong Password" id="password">
                    </div>
                    <div class="form-group">
                        <div class="g-recaptcha btn-block" data-sitekey="6Lec_iYTAAAAAMY6oTP9TakaiFfKQR1lknkLJuP0"></div>
                    </div>
                    <div class="form-group">
                        <input type="submit" class="btn btn-block btn-danger input-sm" id="signup" name="submit">
                        <div id="message" class="mg-top-20">
                        </div>
                    </div>
                </form>
            </div>

        </div>
    </div>
</div>

这是ReCaptcha的Php页面代码

 if(isset($_POST['submit']))
             {
             $username = $_POST["username"];
             $email = $_POST["email"];
             $password = $_POST["password"];
             $privatekey="6Lec_iYTAAAAAPATSFJ-foBWWzOJ2TKT9OSBjr15";
             $captcha=$_POST['g-recaptcha-response'];


               if(!$captcha)
              {
              echo '<h2>Please check the the captcha form.</h2>';
              exit;
         }


$response=file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=YOUR SECRET KEY&response=".$captcha."&remoteip=".$_SERVER['REMOTE_ADDR']);
        if($response.success==false)
        {
          echo '<h2>You are spammer ! Get the @$%K out</h2>';
        }else
        {
           include 'connection.php'; 
           $insert = "INSERT INTO users (username, email, password, group_id)
           VALUES ('$username', '$email', '$password','2')";
           mysql_query($insert) or die(mysql_error());

            $select="SELECT * from users where username='$username'";
                           $selectresults = mysql_query($select);    
                           $row = mysql_fetch_array($selectresults);
             $userid=$row['user_id'];
             $insert = "INSERT INTO applicant_particular (email,user_id,quota_id)
           VALUES ('$email','$userid','24')";
             mysql_query($insert) or die(mysql_error());

            $insert = "INSERT INTO status(status,user_id,year)
           VALUES ('0','$userid','0')";
            mysql_query($insert) or die(mysql_error());
            header("Location: index.php");
         }
          }

这是我要用于JQuery的{​​{1}}脚本，我想用它来检查

AJAX

这是我的emailcheking.php页面

    <script>
    $(document).ready(function () {
        $('#email').blur(function () {
            var email = $(this).val();
            $.ajax({
                url: "email_cheking.php",
                method: "POST",
                data: {
                    email: email
                },
                dataType: "text",
                success: function (html) {
                    $('#emailmessage').html(html);
                }
            });
        });
    });
  </script> 

现在建议我该怎么办？