Difference among sleep() and usleep() in PHP

sleep() allows your code to sleep in seconds.

• sleep(5); // sleeps for 5 seconds

usleep() allows your code with respect to microseconds.

• usleep(2500000); // sleeps for 2.5 seconds

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.