Core principles to make performant PHP applications

Some core principles for 2013/2014 to performant PHP applications:

1. Upgrade to PHP 5.5 with Zend OpCache or APC using PHP-PFM + Nginx
2. Stay up to date with your framework + dependencies (using Composer)
3. Optimize your session store to use signed cookies or database with caching
4. Cache your database and web service access with Memcache, DBA or Redis
5. Do blocking work in the background with queues and tasks using Resque or Gearman
6. Use HTTP caching and a reverse proxy cache like Varnish
7. Profile code with Xdebug + Webgrind and monitor production performance