实时重复的电子邮件检查不适用于jquery

Question

我有一个电子邮件输入字段。我需要检查输入电子邮件是否已存在于数据库中。我已经为它编写了以下代码。

HTML：

<script type="text/javascript" src="emailcheck.js"></script>
<script type="text/javascript" src="jquery.js"></script>
<tr><td></td><td><input type="text" name="email" value="" placeholder="Your Email Address" id="email" required ><img id="tick" src="tick.png" width="16" height="16"/>
<img id="cross" src="cross.png" width="16" height="16"/></td></tr>

jquery就在这里：

$(document).ready(function(){
                                $('#email').keyup(email_check);
                                });

                                function email_check(){ 
                                var email = $('#email').val();
                                if(email == "" || email.length < 4){
                                $('#email').css('border', '3px #CCC solid');
                                $('#tick').hide();
                                }else{

                                jQuery.ajax({
                                   type: "POST",
                                   url: "check.php",
                                   data: 'email='+ email,
                                   cache: false,
                                   success: function(response){
                                if(response == 1){
                                    $('#email').css('border', '3px #C33 solid');    
                                    $('#tick').hide();
                                    $('#cross').fadeIn();
                                    }else{
                                    $('#email').css('border', '3px #090 solid');
                                    $('#cross').hide();
                                    $('#tick').fadeIn();
                                         }

                                }
                                });
                                }



                                }

文件check.php在这里：

<?php
mysql_connect("localhost","root","");
mysql_select_db("ignite");
$email = strtolower($_POST['email']);
$email = mysql_real_escape_string($email);

$query = "SELECT * FROM discover_science WHERE email = '$email' LIMIT 1";
$result = mysql_query($query);
$num = mysql_num_rows($result);

echo $num;
?>

输入电子邮件地址时，代码无响应。问题出在哪儿。请帮帮我。