ขอถามเรื่องมันไม่ยอมเช็ค username ซํ้าในฐานข้อมูล มันข้ามไปลงข้อมูลเลยครับ

$username = $_POST['username'];
    $password = md5($_POST['password']);
    $email = $_POST['email'];
    $first_name = $_POST['first_name'];
    $last_name = $_POST['last_name'];
    $sid = session_id();
    
	 $checkusername = mysql_query("SELECT * FROM users WHERE email = '$email' AND username = '$username'");
     $result = mysql_fetch_array($checkusername);
	 
     if($result > 0)
     {
     	echo "<h1>Error</h1>";
        echo "<p>Sorry, that username is taken. Please go back and try again.</p>";
     }