Recommendations re stable, extensible PHP/mySQL eCom system?

I was wondering if anyone had any advice or recommendations on finding a
stable, extensible eCom cart/ fullfillment system using mySQL and PHP (or
Perl)?



My client is preparing to get online with what might be best described as a
"mall" where it would be helpful to be able to house 5 or 6 stores with
their own stock and fulfillment stuff but also be able to search across
stores, etc.



Any recommendations?



TIA!



jg