Well, this certainly wasn't on my radar. Gábor Hojtsy, Drupal 6 lead maintainer, announced that starting March 1, 2014 support for PHP 4 in Drupal 6 will end. I wasn't surprise to hear about Drupal developers dropping support for PHP 4. Instead, I was in shock to hear that Drupal didn't drop support for this ancient version of PHP sooner.
To put this announcement in perspective, the PHP project developers said their goodbyes to PHP 4 back in 2008 and I personally said my "see ya later" back in 2007. Needless to say, I don't think anyone with merit can complain Drupal is dropping PHP 4 support. In Gábor Hojtsy's words:
Drupal 6.0 was released almost 6 years ago in February 2008. The Drupal community is committed to release Drupal 6 bugfixes until Drupal 8.0 is released and with recent changes provide security fixes much longer.
The hosting and development landscape was very different in 2008 though. PHP has gone a long way since we released Drupal 6. While Drupal 6 is still supported on PHP 4.x, the PHP developer community itself end-of-lifed PHP 4 just half a year after Drupal 6.0 came out. According to public statistics and data available to us about Drupal 6 sites, we estimate that there is a very small number of Drupal sites which may still run on PHP 4. We also don't believe it is in our best interest to support Drupal 6 on a possibly insecure but definitely unsupported base system, so we discussed and decided to drop support for PHP 4 on March 1st 2014.
Typically, Drupal has dropped support for an older versions of Drupal when a new version of Drupal is released. The expectation was Drupal 6 support would be dropped when Drupal 8 becomes an official release. I suspect the delay in dropping Drupal 6 support is postponed partially due to a change in Drupal 8's new site migration approach. There is a new workflow for site migration that has the potential for site owners to migrate their content not only from Drupal 7 to Drupal 8 but also allow Drupal 6 sites to migrate directly into Drupal 8. Until the new migration approach is proven, it is in everyone's interest to continue support for secure Drupal 6 sites. For the "secure" mandate to be supportable no website should be running on PHP 4.
My only question is, how many sites are still running on PHP 4? Given that PHP 4 hasn't been supported for a very long time and all the risks involved, why would you still run your site on PHP 4? Anybody have guts to admit that they're still running PHP 4 applications on a public server and help us understand why they wouldn't migrate to PHP 5?