What operating system should I use for PHP?
NEVER use Windows
PHP support on Windows has always been poor, and if you would like to use a non-standard extension - scratch that, non-standard anything - you'll be out of luck.
ALWAYS use Linux
But which distribution? The most important thing is to standardize: ensure you're running the same distribution everywhere, your development setup included.
Want to pay for commercial support?
Use Redhat Enterprise Linux
Don't want to pay for commercial support?
NEVER use Ubuntu.
You want your servers to be stable and easy to maintain over the long-term.
Unbuntu's focus on the newest and latest is therefore significantly less than ideal for server environments.