Most PHP installations out there are still running PHP 4.x. PHP 5.0 has been out a while, and PHP 5.1.1 has just been released. For those of you who haven't yet upgraded, this month I look at some of the changes you can expect to find in the newer versions of PHP, as well as a preview of what you can look forward to in PHP 6.
They go through PHP5 first, mentioning especially the new object model. PHP 5.1.x is briefly mentioned (especially 5.1.1 just released) but not that many new things were added in that edition. From there, the rest is a look at PHP6 and what we can expect from it - items removed/added to the core, the removal of register_globals, and one of the largest developments: Unicode support.