使用用户名时禁用按钮 jQuery.

我希望能够在用户名被占用时禁止我的表单提交，我想得到一些帮助。

我的JS代码是：

$('#username').keyup(function() {
var username = $(this).val();
$('#username_status').html('Searching...');
if (username !='') {
    $.post('username_check.php', { username: username}, function(data) {
        $('#username_status').html(data);
    });
} else{
    $('#username_status').html('');
}
});

和我的检查用户名是否存在：

<?php
if(isset($_POST["username"]))
{
define("HOST","localhost");
define("USERNAME","root");
define("PASS","");
define("DBNAME","testingproject");
$connecDB = mysqli_connect(HOST, USERNAME, PASS, DBNAME)or die('could not connect to database');

$username =  $_POST["username"];

$results = mysqli_query($connecDB,"SELECT id FROM attendant WHERE username='$username'");
$username_exist = mysqli_num_rows($results); //records count

if($username_exist) {
  $output= "Sorry, this Username is taken";
  echo ($output);
}else{
  echo('available');
 }
}
?>