Monotonic
A monotonic value only ever moves in one direction: it is either always non-decreasing or always non-increasing, and it never goes backward.
Applied to time, a monotonic clock is a time source that cannot jump backward or forward because of NTP synchronization, manual clock changes, or Daylight Saving Time. PHP exposes such a clock through hrtime(), unlike microtime() or time(), which read the wall clock and can be affected by these adjustments. Measuring an elapsed duration should always rely on a monotonic source, otherwise the computed duration could turn out negative.
Applied to identifiers or sequences, monotonic means each new value is guaranteed to be greater than the previous one, a property required by auto-increment database columns, ULIDs, and Snowflake-style distributed IDs.
<?php
$timeStart = hrtime(true);
// some work happens here, even if the system clock is adjusted
usleep(100);
$timeEnd = hrtime(true);
// always positive, thanks to the monotonic clock
$elapsedNanoseconds = $timeEnd - $timeStart;
?>