First of all, how it started isn't as important as where it is now. And what we have now is a full-fledged programming language. Secondly, if you can build large-scale maintainable websites with PHP, who cares how the language got started originally. It's all about what you can do with the language - and in that regard PHP is a great tool (although certainly not the only tool available).
"I don't know how to stop it, there was never any intent to write a programming language [...] I have absolutely no idea how to write a programming language, I just kept adding the next logical step on the way." -- Rasmussen Lerdorf, inventor of PHP
But to your point, he also said: "I've never thought of PHP as more than a simple tool to solve problems."
