jquery validator remote
$( "#myform" ).validate({
  rules: {
    email: {
      required: true,
      email: true,
      remote: {
        url: "check-email.php",
        type: "post",
        data: {
          username: function() {
            return $( "#username" ).val();
          }
        }
      }
    }
  }
});
