PHP在注册表单上重复使用用户名

Question

Trying to stop users from registering duplicate accounts but code doesn't seem to work, don't know what I am doing wrong,

var_dump count shows "string(1) "0"

        $query1 = "SELECT COUNT(*) FROM users WHERE user_email = :email OR user_name = :username";
        $stmt = $DBH->prepare($query1);
        $stmt->execute(array(':username' => $username,':email' => $email));
        $count = $stmt->fetchColumn();

        // var_dump($count);

        if ($count > 0) {
            $error = "Username or E-Mail in Use. Please try another.";
        }