I'm guessing your definition of pain is different then mine - I don't have any issues with writing PHP - it just works for me. I've been developing in PHP since 2002, so by now I'm used to the language and how it works, and don't really have any expectations for new features.
I can see however that coming from a language like Python or Ruby, you're probably going to try to develop in PHP like you would in one of those languages - and I believe that is where frustration will be found. I think it's good to think of PHP as a "simpler" language - stick with the basics and it will work well (as well as be pretty fast).
"...our target support is PHP 4.3.0, so this leads to some creative (i.e. fun) problems at times."
"I don't have any issues with writing PHP - it just works for me."
PHP fits an important niche. People sometimes use PHP because it is faster and has a smaller memory footprint than Python or Ruby.
I was considering language features separately. If you're trying to pick a language, no one would pick PHP unless they need to work with people who already know PHP. If you're trying to pick a runtime, PHP occasionally makes sense due to ease of deployment, memory issues, etc.
"PHP is as "complete" as it needs to be for us."
It's so complete that it's painful to write in? There's something to be said for languages that focus on a particular application while peripheral features suffer, but the PHP language is in no way superior to Python or Ruby, even in regards to web development, and there's no problem with saying so.
People use PHP because lots of other people know it, and because it's widely available. Those aren't merits of the language itself. Yes, good things can be written in PHP. That doesn't mean the language isn't demonstrably inferior.