Всё сломалось!

Класс: DB

engine::run();
engine::getCart();
DB::execSQL("SELECT * FROM cart WHERE order_id="" AND sid="gpdd9mgcv5vce7j9ekd7dc3enc"");

SELECT * FROM cart WHERE order_id="" AND sid="gpdd9mgcv5vce7j9ekd7dc3enc"
MySQL server has gone away