If you want to run Symfony 3.1 on PHP 7 locally, XAMPP is a natural choice. But beware: The XAMPP version 7.0.6 for OS X (the latest PHP 7 based version available today) is broken. It’s so broken that it installs, but Apache doesn’t even start.
Luckily, version 7.0.5 works perfectly, you can download it from sourceforge.net here.