Ecommerce help (finding right script)

I'm looking for a script (much like www.cartit.com) but in php, that allows me to add snippets of php code to my already designed php pages to create a functional website that sells products.

I would use cartit, but 1) it doesnt have a database backend (as far as i can tell) 2) im fluent in php, cartit is in asp/cgi

Also, another option..I have coded my own shopping cart system (php/mysql) the past for a project, how difficult, and what steps would I need to make it function in the real world?