As most PHP developers (and companies looking at using PHP) can tell from some of the recent stats, PHP5's adoption rate has been pretty slow ever since it was released. A lot of PHP applications have been written to use the PHP 4 series, but most would agree that a move up would be a good thing. So, why haven't they? Denis de Bernardy wonders about this too in his latest blog posting.
If you think php5 is a no brainer choice since it's been around for almost three years now, you might want to think again.
I can find lots of reasons to develop in php5 rather than php4. php5 is closer to what I'd expect from an object oriented language, and it's reportedly 25% faster. Not to mention the pdo class, which is wonderful to work with.
Then again, developing software that 50% of your user base simply cannot use doesn't make any sense.
He backs up the point with some statistics that Semiologic collected recently, showing that the PHP 4.4.x series is a clear winner on usage. If a change that wasn't so dramatic (PHP4 to PHP5) has this kind of effect on the version usage, what kind of adoption rate will we see when PHP 6 comes around?