I think the difference in brain cycles spent for a master in OOP vs a master in FP is far less than the difference between a beginner in either compared to the masters of their style.

At least in my experience, OOP has more concepts to learn but each learning step is smaller. FP has less concepts in total but the learning curve has some rather steep parts in it.

PHP is too hard for me. I can't keep track of all the things I need to keep track of not to write unmaintainable, error-prone, insecure garbage. With typed functional languages I do a little better.