Hi,
well if you have a PHP page which frequently gets hits, you could program something like this.
For example: you have a script that you want to daily execute. You would need a database and a page that gets executed for than once every day. Every time a user opens that page the script looks up a value in the database, which tells it when your script was executed last. If that time is older than 24 hours, than it will execute the script now.
wizkid