Incrementing a page link, refresh problem

I write a session to determine what module a user is on $_SESSION['module_no']==1; when they get to the end of the module I want to create a dynamic link to the next module $increment = $_SESSION['module_no']++; $link= "../module_$increment.php"; The problem is every time the page is refreshed $increment is added to. Any ideas or ways around this. I do not want to create an 'end_of module.php' page for every module. R.