Wordpress - What is the best php version to use with WordPress?

IMPORTANT EDIT

This has unknowingly came to my attention as it slipped my mind, PHP 5.4 have already reach EOL and the last security support was stopped on the 14th of September 2015.

According to official documentation, PHP 5.5 will finally reach its EOL on the 10 of July 2016 (Active support has already been stopped but this version will still get security updates until its final EOl date of 10 of July 2016).

So to come back to the original question, you should be running on minimum at least PHP 5.5 now, but be sure to get ready to switch to PHP 5.6 within the next 8 months as from date of this answer

ORIGINAL ANSWER

Wordpress are still stuck on the dinosaur PHP versions, so any PHP version newer or equal to 5.3 should do.

It is however very very important to note that all PHP versions prior to version 5.4 have been EOL'ed, the last version being 5.3 a year ago. In short, this means than any PHP version older than 5.4 are no longer supported or updated, which will cause huge security issues if you are still using it.

So, for safety, the bare minimum you should be running safely is PHP 5.4.x, where x should be the latest security release.

In short, to be rather safe than sorry, just like Wordpress, it is always always the best to have the latest available stable release installed.

To run WordPress we recommend your host supports:

PHP version 5.6 or greater

MySQL version 5.5 or greater

Note: If you are in a legacy environment where you only have older PHP or MySQL versions, WordPress also works with PHP 5.2.4+ and MySQL 5.0+, but these versions have reached official End Of Life and as such may expose your site to security vulnerabilities.

For details please follow code link:

Wordpress Requirements

Thanks!

All the answers here don't take into account an actual, functioning Wordpress environment, but use arguments about end-of-life/support (that is, age). Who cares? What one really wants is stability and functionality. Therefore, one should use the latest version that an entire Wordpress system (with desired themes and plugins) will support. Latest because PHP generally has speed (and stability) improvements in newer versions, but not "the" latest, because stability may suffer.

There are still many plugins (especially those not updated recently) which have deprecated or unsupported PHP code in them, so running the latest and greatest PHP version will not work with those plugins (and themes). 5.6 is the safest, latest version with wide support (90+%), and 7.x is the very latest, and definitely has performance benefits, but a lot of wordpress themes and plugins will not function. Best is to test out your current system and see what your entire environment can work with. Try the PHP Compatibility Checker. It isn't 100% accurate, but gives you something to start with. https://wordpress.org/plugins/php-compatibility-checker/

Finally, PHP versions may have a needed or desired functionality, such as OpCache or php-fpm/mpm-event. In these cases, 5.5 and 5.6 will do.