Difficulty of small project

I'm not very good with PHP, but a forum I help out when I can needs an off-site hosted script through PHP.

It's job will be to make a task where every x minutes to scan through the most recent x entries of posts that has members being quoted from posts table then insert notification in the notifications table.

Translation: When someone is quoted they get a notifcation via PM.

We don't have root/DB access and Ajax isn't possible, so i'm wondering how difficult this would be for someone like me who doesn't know much about PHP

I'd say this is on the mid end... to run something every few minutes requires something called a cron job. It requires having access to the server to setup.

After that, the actual script would be pretty easy as long as you have the access information for the database and the database allows foreign connections (a lot of databases don't allow connections from outside the server they reside on). If your DB doesn't allow foreign connections, there are other methods, but more convoluted.

Why not edit the code that sends the PM? I can understand concerns about upgrades, but as long as you keep track of what you're doing, it may the best way if you don't have a great deal of access to the server.