PHP mail when inactive user

So I'm making a web system, the idea is when a user closes the browser it will send an E-mail to me. Currently the user is using Javascript Ajax to get PHP to update the database with the current time. When the time exceeds 5 mins then I want PHP to send me an E-mail. The database is setup for the users and stuff.

Could you please tell me if it's possible for the server to check every 30 seconds or so for the users to be active or not and send an E-mail if not. and If so could you give some examples of code and such of what I would need to do, currently I don't have access to the server as it's a DEV project at the moment and it's on a free webhosting service. I can move it onto a linux server that I have SSH access to if needed.

I do have a lot of knowlege on PHP but this completely beats me with a stick.

Cheers for any help.