My objections don't start (or end) with PHP. Browsers are terrible platforms, and the protocols we have to deal with are likewise terrible (ever written an HTTP proxy? Oh boy).
Oh yeah, and it's not fairly hard. After all, it's a well defined, text-based protocol. It's sweet to implement if you're not using C. That is, before you started supporting persistent connections or interacting with the content.
In any case, it's loads better than FTP and SMTP (at least HTTP spelled things completely), and loads easier to use than binary protocols.
