That's actually really funny that you asked that. Just this weekend I went back onto my projecteuler account (ask LoG, we were doing some code comparison on one of the missions). Last time I was on PE, I was pretty much coding strictly in Perl and did a small amount of missions. Then coming back to it a year or two later, I figured I'd code everything in PHP. So that's exactly what I did. I did about nine or so missions that night. That said, I did use PHP. Which obviously isn't gonna be as quick and efficient as C or C++, but it does the job just fine. With this, I absolutely base the time and efficiency off the code itself though. Especially when it comes to working with large numbers and long loops.
I do wish I would try to brush up on my C knowledge, as it would be a lot better for things like this. While I was coding a couple of them, I really wanted to keep everything as efficient as possible. And running a compiled language would help just that much more. But I do enjoy my PHP and it was working pretty well for me.
pretentious, if you're getting back into project euler, you should hop on IRC (#coffeesh0p) and bull shit with us. I'd be down to do some coding with you (or anyone else for that matter).
But yes: favorite language changed from Perl to PHP.
For those about to hack, I salute you.