Probably a majority, but not all, are done in PHP. There are many many cart systems done in PHP. Most of them were developed with some type of traditional SQL engine, the most common being MySQL. However, they can be adapted to making FileMaker PHP calls. The reality is that it is usually easier to just use the cart system and back end databse it supports. If it is a database engine supoprted by FileMaker, you are good to go for an easy integration, especially if it is one supported by ESS such as MySQL, MS SQL or Oracle. You can make the ODBC connection and see the live cart data in FileMaker. This is by far the most common way I have seen cart systems integrated with FileMaker.
FileMaker itself can process credit cards and be a cart system with its PHP engine, but all of these I have seen are custom solutions. I work with two clients who have done this. I am just the FileMaker developer and they have a web and PHP developer that connects the cart system to FileMaker's PHP API. One downside to this is that FileMaker's PHP API is as fast as PHP connecting to most SQL database engines like MySQL. This has to do with some things that have not been optimized in FileMaker's API such as making multiple calls at a time in a batch call, etc. So FileMaker's PHP API works, it just is not a heavy duty commercial PHP solution. That is why many people use the scenario in the first paragraph for the cart systems.
What I hear people talk about a lot, but I haven't seen in a live FileMaker solution, is using WebDirect for the cart system. This is probably good for a light duty cart system that is easy to set up quickly. However, WebDirect has a number of limitations including needing to beef up the server and being limited to 50 sessions at a time. But this is where I see some exciting frontiers for FileMaker. Among other things, it combines the security into one system instead of having to separately manage the UI security and the database security.
If you want to contact me off line, I can tell you some PHP cart solutions that are directlly connecting to FileMaker's API, but I just need to get the clients approval first. Most do them this behind the scenes so that no one knows it is FileMaker running it.
We're in the midst of integrating our FileMaker solution, which handles product and customer info, together with the importing of web site orders, their invoicing and payment via credit cards with a new web site, where we will be uploading product info via an ESS connection to the mySQL files on the web server.
Finding a web developer who was prepared to work with FileMaker for data input and output via ESS was initially quite a challenge but once we found the right person, it has been relatively straightforward.
Let me know if you need to know more about the way we've gone about our project.
I concur with John particularly on the point that it is much easier to find PHP developers that are familiar with making PHP SQL calls to MySQL than FileMaker. This is a real plus for using a MySQL cart system since FileMaker can then ESS the data.
and there is a big additional benefit in that you separate the database that is exposed to the web from the rest of the solution
Are you using FP13 webdirect? I like the idea of this (I figure I will be fine with the 50 max for now) but am concerned of speed and latency. I do not want anyone to notice any differences from my site to another site in that respect.