PHP 5.5.7, PHP 5.4.23 and PHP 2.3.28 have been just released (see here)

I am using always the latest version of PHP 5.5.x for all testing/development to make sure that there are no obvious PHP-related bugs, as the PHP5.5.x is much less forgiving and shows more bugs than PHP 5.3.x would.

So if you're a developer, I highly recommend to use PHP 5.5.x for all your development and testing.

Note: Do NOT use any PHP 5.5.x or PHP 5.4.x specific features as our minimum supported PHP version for XOOPS 2.5.6+ is PHP 5.3.7.