First an answer to your question.

No, you probably don't need a flock. On Unix I can guarantee that you don't if your strategy for rewriting the file is to write it to a different name and then rename the new one on top of the old. Otherwise the worst that can happen is a low likelyhood of a page from time to time which is somewhat truncated. (If you do flock, make it a shared lock.)

However you do have many other things about your coding style that could be worked on. For instance a function with return in the name suggests that you are returning stuff. You are not, so the name should better reflect what you are doing. Also it is good to break out argument processing so that it is obvious from a glance what arguments your subroutine takes. And it would be good to do more separation of content and presentation. Aggressively inlined HTML makes both program logic and layout harder to see and manipulate...