Difference among sleep() and usleep() in PHP

Question

Can any body explain me what is the difference among sleep() and usleep() in PHP.

I have directed to use following scripts to do chat appli

Answer 1

The argument to sleep is seconds, the argument to usleep is microseconds. Other than that, I think they're identical.

sleep($n) == usleep($n * 1000000)

usleep(25000) only sleeps for 0.025 seconds.