Server side timer to execute PHP code upon expiration? [closed]

I’m looking for a server side timer that would execute a PHP file at expiration. The timer should be controlled by another PHP file to start, stop and reset the timer.

The PHP module EvTimer would work great in this situation, but it is not available on the Bluehost shared servers I’m using.
Cron jobs? Hmmm, could those be activated when the command is received, canceled when successful?
Other options?