JQuery.Validate() - remote rule [SOLVED]
<form id="regform" class="well" method="post" action="Registering.php">
        <label>Name: <input type="text" placeholder="Type your name" name="Name" class="required" /></label>
        <label>Surname: <input type="text" placeholder="Type your surname" name="Surname" class="required" /></label>
        <label>Username: <input type="text" placeholder="Type your username" name="Username" class="required" /></label>
        <label>Password: <input type="password" placeholder="Type your password" name="Password" class="required" /></label>
        <label>Repeat Password: <input type="password" placeholder="Confirm Password" name="Password-confirm" class="required" /></label>
        <label>E-Mail: <input type="Email" placeholder="Type your Email" name="Email" class="required email" /></label>
        <label>Birthday: <input type="Date" name="Birthday" /></label>
        <label>Gender:</label>
        <label>Male <input type="radio" name="Gender" value="0" checked> Female <input type="radio" name="Gender" value="1"></label>
        <label>Photo: <input type="url" placeholder="Paste an image link" name="PhotoLink" class="url" /></label>
        <label><input type="submit" class="btn btn-primary" name="Submit" value="Register in YouTune" /></label>
 </form>

$(document).ready(function(){
        $("#regform").validate({
            rules: {
                Username: {
                    remote: { url: "/check_username.php", async: false }
                }
            },
            messages: {
                Username: { remote: "Username already on use. Pick a different one." }
            }
        });
});

<?php error_reporting(E_ALL);
     require_once("../Clases/User.php");
     header('Content-type: application/json');
     $vUser = new User();
     $vResult = $vUser->GetUserByUsername($_REQUEST["Username"]);
     if ($vResult) {
    echo false;
     } else { echo true; }