PHP's strength is that it runs well in a www environment. It was not designed to build standalone applications. HOWEVER, the PHPGTK package will apparently allow you to build standalone apps. I've never tried it. PHP is cross platform and when used with a suitable web server is multi-user friendly.
VB has its own quirks (like it won't run on a Mac, for one). But it integrates well with other MS products like Excel and Access. BUT! Neither Access or Excell is suitable for a LARGE project (especially if you intend to have many users concurrently).
It sounds like you need Java or C++, but I don't know much about those. :-) I guess it ultimately depends on your environment and what you are more comfortable with.:p