未捕获错误:无法调用抽象方法(PHP 致命错误）

我正在扩展一个具有两个抽象方法的抽象类。当我尝试实例化子类时，出现以下错误，

 PHP Fatal error:  Uncaught Error: Cannot call abstract method GetAnswer::GetAnswer()

当我注释掉父子GetAnswer()时，事情都很好。

PHP 版本：XAMPP 中的 7.0.2，运行在 Wondows 8.1 上。

父类文件 app-getAnswer-base.php

<?php
abstract class GetAnswer{
    abstract public function GetAnswer($Username, $QuestionnaireID, $QuestionID);
    abstract public function GetAnswers($Username, $QuestionnaireID);
}

子类文件app-getAnswer-db-direct.php如下所示

<?php
require_once 'config-db-login.php';
require_once 'app-getAnswer-base.php';
class GetAnswer_SQLSelect extends GetAnswer {
    public function GetAnswer($Username, $QuestionnaireID, $QuestionID){
        return null;
    }    
    public function GetAnswers($Username, $QuestionnaireID){
        $connection = new mysqli(dbLogin::$db_hostname, dbLogin::$db_username, dbLogin::$db_password, dbLogin::$db_database);
        if ($connection->connect_error) die($connection->connect_error);
        $query = "SELECT `QuestionID`, `Answer` FROM `answerstore`.`answers` AS a JOIN  `answerstore`.`participants` AS b ON a.`ParticipantID` = b.`ParticipantID` WHERE a.`QuestionnaireID` = '$QuestionnaireID' AND b.`Username`='$Username'";
        $result = $connection->query($query);
        $connection->close();
        return $result;    
    }
}