There seems to be a bit of chatter around at the moment regarding php5, now that the end of php4's development lifecycle has been announced.

Why has the adoption of php5 been so slow? There are a lot of theories, but I think perhaps one of the main issues is the target market php was intended for. It is a very "forgiving" language, designed to make web-programming easy. To php's credit, it was so well designed as a web-language, that it could easily be applied to much larger applications and scale very well, compared to more "professional" oriented languages (asp/jsp, etc were aimed more at business solutions).

The newest developments are now bringing php inline with some of the very powerful features found in higher level languages, however your casual php coder has no use for these and is quite happy plodding around in procedural code to "get the job done", rather than find the perfect scalable solution built upon a complex framework.

For those of us who code PHP for a living, version 5 is a godsend in many ways, and most of us already employ frameworks, design patterns and OOP in our coding, so this is simply the next logical step. Your average webmaster who "dabbles" in php, has no need for these sorts of things, and thus - there is no pressure from them on their hosting providers to upgrade. It doesn't actually offer much in the way of amazing new features for the casual user either, which means the desire to upgrade is almost non existent.

In summary, I think the push to get out of the php4 rut is a good one, but there are some underlying business and personal issues that explain why it's not taking on very fast.