Hello,
I'm Emanuel and I'm working for a German company and was in several web based projects. Your project sounds very interesting and so I want to work for you. But I have a question: So far I unterstand your problem you want a Visual Studio application where the user can register themselve for your php-sites. The data should be stored in an MySql Database. But please tell me why! In my opion it doesn't make any sense to connect php and visual studio because both languages are able to do the same, so it is not necessary to combine them. You can have an easier/cleaner solution based only on php.
Kind regards
Emanuel