PHP Script/Thread ID thingie

I'm logging things with error_log, and would like to be able to sort out one script run from another. So I'm looking for some kind of "script id" or "thread id" or "PHP script run execution ID" type of function. getmypid() just returns the same apache child process ID all the time, so that's not what I want. zend_thread_id() looks useful, but I suspect it's not quite what I'm looking for. But I'd have to re-compile with ZTS and --debug-mode and I don't think the function I'm looking for should require that... Perhaps I've just missed the right function name? Or perhaps this should be a "Feature Request"?