Matt of WordPress fame gives his opinion on PHP4 and the transition to PHP5. As he says:
None of the most requested features for WordPress would be any easier (or harder) if they were written for PHP 4 or 5 or Python. Theyâ€™d just be different. The hard part usually has little to do with the underlying server-side language.
Very true. Most of our code continues to run fine on both PHP 4 and 5, with hardly any checking of PHP_VERSION. Migrating from PHP4 to PHP5 has been relatively painless (each time we port an application, we spend at most half a day fixing warnings that didn't appear in PHP4).
Matt asks why the takeup of PHP5 been so low, and is quite disparaging to the PHP internals devs. I don't see it in such black and white terms. PHP5 never had a feature that was must-have or to-die-for. In fact, if you look at PHP's recent changes, most of them are performance improvements, or fixing past mistakes (adding proper date support for example in 5.2.1), or feature tweaks (iterators, etc). Given that most PHP4 developers have found workarounds to things fixed in PHP5, migrating to PHP5 is probably a low priority.
Also some fixes in PHP5 can cause serious problems. For example, In PHP 5.2.0, time-zone calculations started to support epochs, which are time-changing events. Now my timezone was +7.30 GMT until 1980, when we got a new Prime Minister who decreed that the country would combine its 2 time-zones to 1, so Malaysia standardised on +8.00 GMT. A timestamp such as "12.10am Nov 12, 1979 MYT" in PHP 5.1 would be displayed as "11.40pm Nov 11, 1979 MYT" in PHP 5.2.
On a more optimistic note, as long as the market share of PHP remains strong, I think the take-up rate of PHP6 will probably be higher than PHP5 in the non-English world, simply because managing multi-lingual and sites that don't use ASCII for their native script is much easier in Unicode. Now that's a compelling reason!