验证现有用户

I have a problem with the validation of an existing user in the database. I program using PHP.
My method code follows:

public function isRegistry(){
    $isUser = $this->dbHandler->prepare("SELECT * FROM `users` WHERE `nickname` = BINARY :nickname");
    $isUser->bindValue(':nickname', $this->nickname, PDO::PARAM_STR);
    if($isUser->execute() == false) {
        print_r($isUser->errorInfo());
        return false;
    } else {
        return $isUser->fetchAll(PDO::FETCH_ASSOC);
    }
    return true;
}

I wonder how to check whether the user exists in the database.

public function isRegistered($nickname){
    $sql = "SELECT 1 FROM `users` WHERE `nickname` = BINARY ?";
    $stmt = $this->dbHandler->prepare($sql);
    $stmt->execute(array($nickname));
    return $stmt->fetchColumn();
}

Use PDO::fetchColumn()

Check out this link (example 2) for more information.