You can create your own API (Application Programming Interface) that the other sites will access and process before opening pages.
I sort of like the API method because you can give the other people a token key,
and then, when the script is called, you'll know which site is requesting their status.
Let's say you have an invoice type of system that detects my payment is due.
You add that to a MySQL table. When my admin panel opens, it calls your API
and your script sends me back a message that my payment is due.
I would include a small script at the top of my admin page that uses CURL
to request data from your site using a URL like this, or an XML request ...
The response I expect back is an XML response ... that I can parse out the message.
Your script called "check.php" sees my token key and queries your database.
If it sees a pending message, it sends back an XML response.
I looked for an easy API tutorial ... this one is from 2005, so it's sort of old:
Maybe someone else has a newer PHP Class or method to make handling easy.