Ahh, I think I understand, you want to be able to set the crontab from within php (or do something to that effect). Well, if there's a possibility that there will be lots of crontab entrys (which seems to be the case with your scenario) I'd create a little app which controlls these and then just run that app on the crontab at pretty close intervals. Two advantages in this
1. You don't clutter up your crontab. I've got about 30 entries in my crontab most of which are controll apps, if I didn't have these controll apps and just had straight entries in the crontab I'm sure I'd have well over a thousand!!
2. You get a lot more controll over the situation. If you stored all the details in a database you easily do some nice reporting on which scripts are being run when, you could also add extra parameters to scripts if they're being run at certain times.
If you need some help with the specifics please post back with a bit more explination on your problem.
HTH
Bubble