title: PHP OPcache optimization levels - what are they?
body: <p>There is an <a href="http://cz2.php.net/manual/en/opcache.configuration.php#ini.opcache.optimization-level">opcache.optimization_level</a> php.ini directive. It is a bitmask a defaults to <code>0xffffffff</code> - so by default OPcache does all the optimizations.</p>
  
  <p>What kind of optimizations does OPcache do? What passes on bytecode are done?</p>
  
  <p>Follow-up question: is there a code pattern that OPcache can optimise very well? For example, HHVM can skip execution of files that contain only class/function declarations and it just fills class/function tables.</p>