i want to create cart system in php. it need maintain invoice number. customer select many items each time they click "add to cart". When they click "finish purchase" then only invoice number wil be increment. how to do this..

You're asking an enormous question here, and I don't think you'll find a ready answer.

Shopping carts are complex applications with significant security implications. If you're not familiar with how to code them you may be better off finding an existing shopping card system to use. There are many commercial and open-source solutions out there.

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