qantities are not writted in the db

hello, I don't know codeigniter. The dev is gone away and the customer asked me to change anything into the css and to add a line total for each order in a table. I did it directly in the php files. And it works. But when I tested the cart I seen that the quantities are not saved when I confirm the cart. I asked at a friend to test it and the first time he did it with the same account, it worked and I seen the good quantities in the db. But if he make a new order, quantities are not saved. And in the db all qty are again 1.
is it possible to correct it directly in a php file and wich one ?
I hope I'm understandable I'm frenche :-)
Thanks a lot for your help !