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

usleep() is used to delay execution in "microseconds" while sleep() is used to delay execution in seconds. So usleep(25000) is 0.025 seconds.

Is there any difference between the two?