The eCart tools produces the commerce and cart functionality and will work within a php based site. There is nothing within eCart that prevents it from being added to an existing site. You can run into some problems though depending on how your site is setup and works, without having the experience of adding in eCart to a CMS system like you have it is difficult to say what specifically will be needed to get it working for you the way you would like it. If you have the eCart tool already you should start off by making a copy of your CMS site and adding eCart to the copy. As you have noted there is some documentation on the support pages for using eCart, please post back with any specific questions that you have about this process and we will be glad to help you with whatever we can.