<?php
    class MySQL_Connection{
        protected $database;
        protected function __construct($user, $password, $host){
            try{
                $RDBMS = strtolower('MySQL');
                $DBName = strtolower('DBName');
                $this->database = new PDO($RDBMS.":host"."=".$host.";".$DBName."="."PDirectory", $user, $password);
            }catch(PDOException $Exception){
                print $Exception->getMessage();
                exit;
            }
        }
        protected function __destruct(){
            $this->database = NULL;
        }
    }
    ?>

<?php
    require 'MySQL/Connect.php';

    class MySQL_Login extends MySQL_Connection{
        private $QueryResult;
        public function __construct($user, $password, $host = 'localhost'){
            parent::__construct($user, $password, $host);
        }
        public function __destruct(){
            parent::__destruct();
        }

        public function MySQL_Query($Username, $Password, $PIN, $ID){
            $Password = strtoupper(MD5($Password));
            $PIN = strtoupper(MD5($PIN));
            //Code the giving error -->
            $Query = MySQL_Connection::$connection->prepare("SELECT * FROM Users WHERE Username = ? AND Password = ? AND PIN = ? AND ID = ?");
            //Code the giving error <--
            $Query->execute(array(
                $Username,
                $Password,
                $PIN,
                $ID
            ));

            $row = $Query->fetch(PDO::FETCH_ASSOC);
            if($row['ID'] != NULL){
                $this->QueryResult = $row['ID'];
            }else {
                $this->QueryResult = FALSE;
            }

        }
        public function result() {
            return $QueryResult;
        }
    }
    ?>

PHP Notice:  Accessing static property MySQL_Login::$database as non
static in MySQL/Connect.php on line 17 -

PHP Fatal error:  Uncaught Error: Access to undeclared static property:
MySQL_Connection::$connection in Login.php on line 17 -
Thrown in Login.php on line 17  -