CodeIgniter is a great PHP framework to work with. One of the items several developers have been asking to be included in the next CodeIgniter release, is a core library for a shopping cart. The library is being added for CI version 1.7.2, and is currently available via SVN on the CodeIgniter website.
UPDATE 09/12/2009: The shopping cart library has now been released as part of the Code Igniter stable release version 1.7.2. You can dowload it at CodeIgniter.com