致命错误：在非对象上调用成员函数prepare（）

Question

错误：致命错误：在第10行的/home/u526894334/public_html/includes/class/admin.class.php中调用非对象的成员函数prepare（）

admin.class.php代码：

class admin extends system {

public function login($username,$password) {
    $username = htmlspecialchars($username);
    $password = htmlspecialchars($password);

    $query = $this->PDO->prepare("SELECT `id` FROM `admin` WHERE `username`=? AND `password`=?");
    $query->execute(array($username,$password));

    $count = $query->rowCount();

    if($count == 1) {
        $info = $query->fetch(PDO::FETCH_ASSOC);
        $_SESSION['aid'] = $info['id'];
        header("Location:index.php?id=admin_web_settings");
    }else{
        echo "<div class='alert alert-danger'>Neteisingas slapyvardis / slaptažodis!</div>";
    }
}

........

system.class.php代码：

class system {

var     $isConnected = false,
        $ConnectionError = false,
        $pdo = null;
public function __construct(){
    try{
        $this->PDO = new PDO('mysql:host=mysql.2freehosting.com; dbname=u526894334_vvp;charset=utf8', 'u526894334_vvp', 'my_password',array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
        $this->isConnected = true;
    }
    catch(PDOException $ex){
        $this->isConnected = false;
        $this->ConnectionError = $ex;
    }

}..........

config.php代码：http://hastebin.com/oxonajohoh.php