What to use ? time() function or $_SERVER['REQUEST_TIME'] ? Which is better?

Which one among these two time functions is better?

I have a form on my website which is submitted by thousands of users every microsecond or less , thus , thousands of requests at the same time on my server.

So I want to use the one which does not uses any sleep while execution. Also , how unique is $_SERVER['REQUEST_TIME'] and time()? Do both change every microsecond?

Will $_SERVER['REQUEST_TIME'] change every time I submit the form? Like , the time() function will change at every time difference.

I need to give a unique url to the user for verification in which am appending the unique result generated from any one of the two? Also, using microtime() would make PHP sleep? isn't it?