To be fair, I don't really have any direct interest in php personally - my personal list of interests would be in this order:
Flash
Javascript
DOM
.
.
.
.
PHP
- but I get the feeling that cart functionality will be a common request, and it's not something I've ever done before so I figured an understanding of PHP would probably be a default requirement for a web privateer.
Still, thanks for the advice. Think I'll tackle Flash first, then worry about JS/PHP decision.