用ajax检查可用性

Question

我想检查用户在带有ajax的表单中输入的“电子邮件”的可用性。所以我写了下面的代码，但没有发生！

我的错误在哪里？

html文件：

   <form id="frmSignUp" name="frmSignUp" method="post" action="signup.php">

        <!-- Some input fields here-->

        <p>
          <label for="email">Your email:</label>
          <input name="email" class="text" id="email" /> <span id="status"> </span>
        </p>


         <p>

          <input type="submit" name="Submit" value="Submit" id="submit" />
         </p>

        <br />

      </form>

      <script type="text/javascript">
        document.getElementById("email").onblur = function() {
            var xmlhttp;
            var uname=document.getElementById("email");

            if (email.value != "") {
                if (window.XMLHttpRequest) {
                    xmlhttp=new XMLHttpRequest();
                }
                else {
                    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
                }

                xmlhttp.onreadystatechange=function() {
                    if (xmlhttp.readyState==4 && xmlhttp.status==200) {
                        document.getElementById("status").innerHTML=xmlhttp.responseText;
                    }
                };
                xmlhttp.open("GET","/email_availability.php?email="+encodeURIComponent(email.value),true);
                xmlhttp.send();
            }
        };
    </script>

email_availability.php包含：

<?php
        require("_includes/constants.php");

        $email=$_REQUEST['email'];
        if( preg_match("/[^a-z0-9]/",$email) ) {
            print "<span style=\"color:red;\">Username contains illegal charaters.</span>";
            exit;
        }
        // 1. Create a database connection
        $connection = mysql_connect(DB_SERVER,DB_USER,DB_PASS);
        mysql_set_charset('utf8',$connection);

        if (!$connection) {
            die("Database connection failed: " . mysql_error());
        }

        // 2. Select a database to use 
        $db_select = mysql_select_db(DB_NAME,$connection);
        if (!$db_select) {
            die("Database selection failed: " . mysql_error());
        }
        $query = "SELECT id FROM users WHERE email = '{$email}'";
        $db_result = mysql_query($query, $connection);
        $record = mysql_fetch_array ( $db_result );

        if( mysql_num_rows($db_result)>0 ) {
            print "<span style=\"color:red;\">Username already exists :(</span>";
        }
        else {
            print "<span style=\"color:green;\">Username is available :)</span>";
        }
?>

我有另一个问题：

在我的注册表单中，如果电子邮件不可用，如何冻结提交按钮？！

请帮助我的朋友......谢谢