PHP 5.6 is about to enter its final beta this week or early next.
It is now in “feature freeze” where only bug fixes will be applied to the code.
While PHP 5.5 was actually SLOWER than PHP 5.4 in some cases, it appears they have finally moved performance ahead with PHP 5.6 and informal benchmarks show it to be up to 10% faster than PHP 5.4 (and 5.5) using real-world code.
There will also be some further memory savings with how it handles strings and other data. Given the sheer size and bloat of modern WordPress, it will need every bit of help that PHP can throw at it, so I expect quick adoption on large websites.
Fortunately there are very few backward incompatible changes with PHP 5.6
The biggest “problem” will be deprecated warnings for users adapting to it from previous versions – but those warnings can be masked for now to reduce noise in error logs.
For example here is how I handle such noise on production servers via php.ini:
error_reporting = E_ALL & ~E_STRICT & ~E_DEPRECATED & ~E_NOTICE
The future of PHP looks quite exciting now with the recent proof-of-concept demonstrated via PHPNG (PHP next generation), where it might become 50% faster in some cases based on internal restructuring, memory handling modeled on jemalloc and just-in-time compiling. WordPress was already demonstrated to be 20% faster in an early experiment over PHP 5.5