I have a ColdFusion website that is currently running well with login/register modules. I want to use my ColdFusion session in PHP. Can this be achieved?
As mentioned in the comments, no it's not really possible. CF and PHP cannot share in memory sessions variables. However, there are other alternatives such as using cookies.
I used that approach once because a client wanted to share sessions with PHP Forum software. I simply grabbed it using cookies, by finding what cookies were being set ie <cfdump var="#cookies#">. Then converted those cookies into sessions variables. (If you need it the other way around, try doing it in reverse.)
Another possibility is sending data securely by using encrypted URL variables. But we need to know more about your goal. Can you explain exactly what you are doing?