PHP - Using $this when not in object context

When I try to execute this code from inherited class I got this error Using $this when not in object context here my code

abstract class Connection {

    private static $host;
    private static $username;
    private static $password;
    private static $database;

    public function __construct() {
        self::$host = 'localhost'; // database server address
        self::$username = 'root'; //database server username;
        self::$password = ''; //database server password;
        self::$database = 'oms'; //database name
    }

    private function connect() {
        return new PDO('mysql:host=' . Connection::$host . ';dbname=' . Connection::$database . '', Connection::$username, Connection::$password);
    }

    protected function execute($sql) {
        $this->connect();
        return $this->connect()->query($sql);
    }

}

what are the reason for that? i don't use any static method in Connection class. SoWhy do give this error?