Here is an interesting problem, over seven years ago PHP 4 was released. It was a vast improvement over 3 and the world adopted it quickly. When PHP 5 was released the move was much slower and is still the minority on web hosts.
I have moved all of my personal PHP projects to 5 a while ago but professionally I am stuck maintaining an application in 4. I am constantly disappointed when I come up with a solution to only realize that the function I want to use is only in 5. (We are going to be moving to 5 later this year).
The crux of the problem is in how many web hosts use Plesk/cPanel/etc and they haven’t moved their default settings to 5.
Now there is a movement for OSS developers to move their projects to only support 5 called “Go PHP5″. Read the thread here