In a Technetra (India) interview, Rasmus explains where PHP 5's additional OO features are coming from:

"There were a lot of user requests for better Object Orientation in PHP. And a lot of the younger kids coming out of school these days want OO. Everything is Object Oriented. I'm old. In the computer science, Internet stuff I'm pretty old. And back when I came out of school, we didn't have any Object Oriented languages. That really wasn't how I thought about stuff. I taught myself how to program in C by basically writing a couple lines of C, compiling it, then disassembling it and looking at the assembly instructions. And then, after a little while you figure out, OK, if I write it this way, this is the assembly I get and it takes this many instructions and this many (machine) cycles. Back then I could look at a piece of code and I could say that's about 87 cycles for this piece of code.

Kids today have no idea. You ask them how many cycles a particular Java method takes and they have absolutely no idea (they can't easily know, of course). In another sense you can be a lot more productive when you don't have to sit and worry about registers and cycles. Still, getting as close to the hardware as you can does make for the most efficient code. It's, of course, a trade off. So all these user requests for better OO have spurred on the changes in PHP 5, but it's not a revolutionary change."