Like sleep(), this function will pause the script’s execution. The difference is that usleep() works in microseconds. One million microseconds equals one second.

The value given must be a non-negative whole number. A negative number will generate a warning.

// Before
echo date('H:i:s');
 
// Sleep 5 seconds
usleep(5000000);
 
// After
echo "\n" . date('H:i:s');
/*
22:00:28
22:00:33
*/
See the manual entry for usleep()