I could be wrong, but it could be a matter of what the users with problems have their Message Editor Interface set to...
If you go to User CP at the top of the page and then click on Edit Options on the left, the very last thing on the page is Message Editor Interface... Basic Editor only gives you a text box... This lets you pick an icon (similar to smileys in appearance), but no smileys... I use Enhanced Editor... It doesn't get any more confusing to use, but if I want to do special formatting, insert pics and/or hyperlinks, or add php (apparently), I can.
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