function micro()
{
   list($usec, $sec) = explode(' ', microtime());
   return $result = ((float)$usec + (float)$sec);
}


$pre = 0;
for($i = 0; $i < 100; $i++)
{
   $pre = micro();
   usleep(100);

   if(($cur = micro()) < $pre)
      printf('$pre = %f <br /> $cur = %f <br /><br />', $pre, $cur);
}