I'm really shocked by this. According to the latest Nexen.net PHP Version Survey, over 80% of the sites out there running PHP are running some flavor of PHP 4.

Why?

For those of you who missed it, PHP 5 was released almost THREE YEARS AGO. That's right ... in a couple months, we'll celebrate the 3 year anniversary of PHP 5.0.0 stable's release.

I am fully aware that there are a handful of compatibility issues between PHP 4 and PHP 5 (outlined here). I was bitten by a few of those myself when I first started porting code over to PHP 5, but none of them were major issues.

So, PHP 4-using majority: what's keeping you?

Is it your ISP that is behind the times? Your company's system administrator? Your dependency on slow-to-upgrade libraries or applications? Or something else?

Please post the reason you haven't upgraded yet in the comments below. I'd very much like to know what the resistance is to PHP 5.