I am not trying to be boastful. There are many software developers who have had similar experiences. We have had long and happy careers creating things, solving problems, connecting things, helping people do their jobs better. The languages, operating systems, frameworks and libraries were all just tools to get the job done. I attribute my long and happy career to a desire to learn all I can, to try new things, and to find the best tools for the job, and the best ways to use them, whatever they might be. I have prioritized having work over working in a particular language/platform.
Limiting yourself to one platform/language will soon limit your opportunities, which I believe is what you are now starting to experience. If I were in your position, I'd take the PHP job and do my best to learn it like a pro. It can only make you better and more marketable to have both .NET and PHP experience.
What will make you successful is learning and applying good software design principles and implementing them in the languages and platforms that make the most sense at the time, working with other talented people and learning from them.