I do actually pick it for several reasons.
1. I want to know if the environment I am going into is pragmatic about languages. If you are going to refuse to continue working with me over something so minor as PHP then we aren't a culture fit.
2. I want to showcase my best. I know this language in and out. It's the first language I learned and I have the greatest ease of building algorithms with it. I've had interviews where I was asked to do something with PHP that PHP did not support and I found ways to make it work.
3. In some ways I love the conversation that comes with positive interviews when using it. Oftentimes the interviewer isn't very familiar and it sparks discussion on the languages benefits and drawbacks further showcasing my skillset.
Sure as we progress along in the interview process I will switch to whatever language the company uses primarily but if I get a choice I always choose PHP.