pocketmine\utils\Binary::readSignedShort PHP Method

    public static function readSignedShort($str)
    {
        if (PHP_INT_SIZE === 8) {
            return unpack("n", $str)[1] << 48 >> 48;
        } else {
            return unpack("n", $str)[1] << 16 >> 16;
        }
    }

 public function getSignedShort()
 {
     return Binary::readSignedShort($this->get(2));
 }