I'd like to breakpoint on built-in PHP session management functions such as session_start().
Is there any way to do this?

Why not to set it on the php code where you call this function?

if on the other hand you need to debug session_start, feel free to download php sources, compile them with debug info and proceed with debugging in C/C++ IDE like visual studio (windows) or kdevelop (linux/unix).

The issue is there are many places in the code where these functions are called. I want to see where they're being invoked from in specific cases.

The wrapper is the best solution, but it's more of a pain then I expected.

I would probably handle this situation by defining all the wrapper functions in one file, setting that file as an auto-include in php.ini, and doing a global search-and-replace on the actual function calls to replace them with calls to the wrappers. Just make absolutely sure that the wrappers are parameter-compatible with their real-function equivalents, lest you create runtime errors.

The biggest downside to this approach is that it slows the performance of those function calls, since it adds a user-level function call into the mix. However, for functions like session_start() that you aren't calling thousands of times in a tight loop, the penalty probably isn't even measurable by the profiler, yet alone noticeable.