PHP4 is apparently going to be supported only until the end of the year. The idea is to push developers towards PHP5. Matt Mullenweg notes that PHP4 is adequate for a lot of developers, but also claims that PHP5 adoption is poor because PHP5 hasn't been marketed properly to developers. I don't believe this. PHP5 is patently a better language, resolving the single most dreadful language problem that PHP4 exhibits: object copying on reference. Expert developers know this, amateur developers are unaware of the problem and use PHP5 with a kind of religious zealotry.
This migration poses a particular challenge. Rarely does a language change so drastically without offering a simple migration strategy. Vast amounts of legacy code simply don't work on PHP5. mod_php4 and mod_php5 don't run in the same Apache instance so it's not trivial to configure a box to serve some sites with PHP4 and some with PHP5. There is no solution that does not require a lot of sysadmin work setting up proxies, or even virtual machines, and of course, this is not the kind of thing distros do out of the box.
PHP developers seem lacking in conscientiousness about the community they are supporting, as attested by this quote from the PHP4 to 5 migration documentation:
Many PHP programmers aren't even aware of the copying quirks of the old object model and, therefore, the majority of PHP applications will work out of the box, or with very few modifications.
In fact, that entire subsection of the documentation carries the tone that compatibility problems are inconsequential. I think it's telling that the PHP documentation can't produce a complete list of reserved keywords. As the user-submitted comments note, there are at least half a dozen missing from the list.
I've also discovered that PHP4 is simply not available for Ubuntu Feisty. While I can understand that there is a genuine desire to move the PHPosphere forward, it's incredibly dumb to gauge whether people are ready to ditch PHP4 by looking at supported, off-the-shelf web applications, rather than considering the volume of cheap legacy applications. Many people simply need both.
For myself, I'm happy to carry out the migration, but it's annoying that it's been handled so badly that my job in doing so is so very much harder. Hard enough that I've already put it off for years.