I have this loop:

//open archive $zip
$start = hrtime(true);
while(true) {
  $zip->addFile($files[$i++]);
  $end = hrtime(true);
  $time = ($end - $start) / 1e+9; // convert to seconds
  if( $time > 10.0) {
    break;
  }
}
echo $i;

I found that the timer does not accurately stops after 10sec. Whereas if I replace the loop with the following:

//open archive $zip
$start = hrtime(true);
while(true) {
  sleep(1); // simulate work

  $end = hrtime(true);
  $time = ($end - $start) / 1e+9; // convert to seconds
  if( $time > 10.0) {
    break;
  }
}
echo $i;

The loop does stop after 10 or more seconds. Is ZipArchive::addFile() doing something to the hardware timer the prevents predicting proper time difference?
I understand I could use microtime(true) but I understand it might be even less accurate in VM environments.