PHP is open source, multi-platform, server-independent tool by design. It scales well, have some community support etc.
.NET products are closed, one-platform (windows), server dependent tools by design, but of course there are many more-or-less compatible open solutions (e.g. mono). .net also scales well, and have constant m$ support.
PHP is a “scripting” language, while .NET is more like Java (but this doesn’t matter really).
In my opinion, both are worth of knowing, both have some pros/cons (no, please don’t flame-war here).
If I were you, I would simply install PHP into your existing M$ server and simply run your already-written PHP code.