Some exciting and promising coming changes in PHP 6 or 7 have been anounced recently by Dmitry Stogov from Zend. A detailed article has been postd here http://news.php.net/php.internals/73888.
Briefly, Zend is working on PHP NG (next generation) which will bring better performance and better memory management. According to Dmitry, the PHP application execution typically takes a significant part of the execution time dealing with memory allocations, and that affects PHP performance significantly as well.
I spent a significant amount of time experimenting with JIT, and even created a PoC of transparent LLVM based JIT compiler embedded into OPCache. The results on bench.php was just amazing – (0.219 seconds against 2.175 – *10 times speedup of PHP 5.5*), but on real-life apps we got just few percent speedup., - says Dmitry in his report.
According to his tests PHP developers can gain up to 20% more requests per second (in case with Wordpress for example).
So far it looks like upgrading to PHP NG should be painless (that's the idea). However, some of PHP extensions wil might require some "massage".
Looking forward to test the new PHP 6. Or maybe 7? ;)