We have a game project designed on ASP&SQL Server, and we need to integrate it's user database with Drupal.
This would be easier from Project to Drupal (since there is user_save and user_delete functions available globally by using drupal bootstrap) but I'm not sure if we can execute PHP functions on an ASP platform.
Is there any documentation for this kind of problems? What do you suggest?
First of all, I think you should rethink your strategy. Why did you choose ASP & MSSQL and why did you choose Drupal?
But if you really want to stick with it I guess by far the easiest way would be to write a PHP (SOAP) webservice that can be used to interact with Drupal (ie. add/delete users) using the Drupal API, and call this from your ASP code.
*edit*
You could use the SOAP Server and Services modules of Drupal. Or just write a plain and simple webservice from scratch using the PHP soap classes.
If you're just looking to use the games user database for authentication I know Drupal has a few modules that allow for authentication via external services. I believe basic or digest HTTP is pretty straightforward but it probably wouldn't be too difficult to write a services layer on the ASP app. Check out Drupal's "user access/authentication" section for ideas on external authentication.