It really depends on the career path you want to follow. Here are some very general guidelines.

If you want to work for one of the Fortune 100 companies (financial, pharma, etc.) you are better off knowing .NET. Many of these companies have systems running on JAVA and .NET, but very few (if any) in PHP.

If on the other hand you wish to pursue a career in any other environment, you can go far with PHP. You will most likely develop more "current" applications instead of supporting and extending legacy code.

There are some cutting edge companies and there are some "safe" companies. Ultimately, you will have to make the decision where you belong. You have a lot of choices; JAVA, .NET, PHP, RUBY, PYTHON and yes, even COBOL (if you want a nice steady paycheck).

~m