Provided your host supports PHP, and I think you’d be hard pressed to find one nowadays that doesn’t, you really ought to be good to go with PHP. Just ensure that you change the page type to PHP, so that its correctly interpreted by the web server and you should be fine.
Presumably you are planning grabbing a bunch of values from a page/form and Ajax’ing them into PHP, doing something with them, then passing value(s) back to the page. Can’t see any issue with that. Course, depending on the complexity there may be no need to bother with PHP at all - javascript gets you a very long way in situations like this and may be easier.
There is virtually nothing ‘limiting’ about using RW - it’s just a different skin stretched over an old ceremony. Integrating external code into RW projects is transformative and in many ways frees you from certain limitations while giving you a great way to learn and develop re-usable skills.
Let me know if you hit any issues and I can try to help, if you like.