PHP Shopping Cart Recommendation

I'd like to get everyone's opinion on a good PHP shopping cart. We need something that will handle a lot of similar items and can be geared for an industrial site selling products from different manufacturers. It needs to be PCI compliant and able to handle at least 5 sub-category levels. I'd also prefer to have it _not_ display the items until after you've drilled down to the final sub-category. The cart needs to be rock solid and should look professional. We're not looking for wizz-bang, look-at-me fancy. We're selling commercial items here, not women's shoes. (My apologies to the women on the list. =;) Real world experience, hearsay, or just gut reactions are all welcome. Thanks for your help, Frank