I haven't touched PHP for a long time now. How is the Unicode support now? I remember having to use some special utilities to handle characters like "œ" properly.
However, you have the multibyte string functions family that can operate in a wide range of encodings (including UTF-8 which is the default in any sane installation nowadays).
Handling those "correctly" with a string length function gets complicated in any language, as there isn't a 1-to-1 mapping between Unicode codepoints and visible glyphs.