œ works fine with mb_strlen(). What might have been tripping you up is combining character sequences:

https://3v4l.org/DM4pC

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.

In PHP grapheme_strlen achieves what you're describing: https://3v4l.org/HPOb3

Yes, I think you nailed what my issue was.