1. In your PHP code, add in a method which will return the count of sessions (link below)
2. Add an API or page in PHP to return the value from step 1.
3. In your Windows form, add a method to call #2.
4. Populate label with contents from #3
This may help you with the PHP Session Count:
php - Can I count sessions to determine number of people online? - Stack Overflow[
^]