Ed Finkler has pointed out a handy resource for those trying to cope with using the UTF-8 support included in several of PHP's functions - this page on the Web Application Component Toolkit wiki.

This page is intended as a reference for functionality PHP provides which can either help with handling UTF-8 or should be regarded as a risk when used in conjunction with UTF-8 encoded strings. Further information can be found on the Internationalization (I18N) and Character Sets / Character Encoding Issues pages.

It talks about the "dangerous" functionality PHP has (issues that the language has in current functions) when using things like the PCRE extension, the string extension, the array methods, handling variables, the XML extensions (DOM and SAX), image manipulation, and URL parsing functionality.