PHP Cron: Parse cron schedules to find when cron job ran

This class can parse cron tab schedules. It does not parse cron tab files themselves, but instead, can determine when a cron job should have last run, given the schedule in cron tab format (like 0 5 * * 1-4)

It can determine when a cron job should have last run, which can be used in conjunction with database logging to determine if a backup cron server should run a job or to create a complete PHP alternative to cron (maybe distributed across several servers). Updated with some example code and documentation explaining how it might be implemented.