Run PHP script automatically without user input at certain conditions?

Use the cron job.

I think a cron job would work fine. If your script runs before the right time, presumably all it needs to do is check the time and exit; that's a very cheap operation and it couldn't hurt you to run it every hour, minute, second, or whatever. I don't even see why you can't just have a script that runs continuously and not even bother with scheduling it at all. If the timing needs to be very precise, and for some reason you're very concerned about running the script often, you could have an infrequently-running script that triggers another script to check the time much more often when the time is getting close. But to me that seems like overkill.

Why do you need the cron job to keep checking the time? Why not just (re-)schedule it for the right time in the first place?

http://ask.metafilter.com/168906/Run-PHP-script-automatically-without-user-input-at-certain-conditions#2428902: "A small script and a single database query per minute is completely negligible in terms of server load." Completely agree. If the "Is there work to be done?" query is worrying you from a performance standpoint, ur doin it wrong. Just make sure there's an index on that field. http://ask.metafilter.com/168906/Run-PHP-script-automatically-without-user-input-at-certain-conditions#2428946: "I don't even see why you can't just have a script that runs continuously and not even bother with scheduling it at all." I assume you mean running as some kind of daemon. Aside from the host provider probably not allowing that, you'd need to build in some kind of handling for when the daemon silently fails for whatever reason. For a simple task like this, a cron job that makes your process live and die each time is more straightforward.

I assume you mean running as some kind of daemon. Aside from the host provider probably not allowing that, you'd need to build in some kind of handling for when the daemon silently fails for whatever reason. For a simple task like this, a cron job that makes your process live and die each time is more straightforward. Yes.

Nthing the cron job. That's not the only way to do it, but in the vast majority of the situations, it's the best.

I would also use cron with either a DB based lock or just flock on the executable itself (or maybe better, its logfile) to prevent multiple simultaneous executions. If you know when you need to run the script next within the script itself I would use at to schedule the next run.

Unless I'm missing something (and I may be because I'm not very familiar with Dreamhost), you should be able to write a tiny shell script (bash? not sure what shell Dreamhost uses) that checks the field in the mysql database (you'd probably need to read the output of the mysql command line call into a variable), compares it to the current time, then kicks off the "tick.php" script if necessary. This should definitely be lightweight enough to run every minute via a cronjob. IMO, the more you can push this sort of operational/maintenance type thing down to the shell itself, the better -- it will be more stable than some higher-level mechanism you build, and it will be easier to troubleshoot.

it will be more stable than some higher-level mechanism you build, and it will be easier to troubleshoot. This is only going to be true if you are just as good at shell scripting as you are at the high-level language of your choice. If you know PHP, use PHP. The performance and stability gains of that kind of shell script over a similar PHP script are trivial at best, and if you're not great with shell scripting, there's a lot larger chance of coding error or edge case you're not familiar with.