Here at work we have a tech that writes the apps for Human Resources with .NET; but we, the web developers, are all on LAMP...
So, they can coexist, but for web-development it is kind of a no brainer; PHP is free, easy to install, comprehensive, and has a truly massive audience. Other alternatives would be Python + Django/(insert hundreds of other frameworks here). I personally like Python + Django the most out of all the language/framework combos; but, the other developers in my work are not familiar with anything Python, so I use PHP + ZF, which I like quite alot anyways. I think you have some pretty good arguments at this point.
