eCommerce with PHP and MySQL

Hi guys, Sorry if this is an old hat question, but I figured its best to speak to those who know best ;o) We're planning to implement a series of eConmerce solutions using PHP with a db backend. Initially our thoughts were to use MySQL as this is what we have (successfully) used before for other (non eCommerce) projects. Are there advantages to using other DB systems (extra features etc.)? Also, and I appreciate this is not really a PHP question so please humour me, how easy (and how would one) is it to integrate automated credit card/transaction proccessing with the PHP solution (obviosuly securely ;o) ) Thanks once again, Tom Carter Web Programmer roundcorners.com