Still, it's gotta be useful to be know how many bytes it occupies. Perhaps
for Content-length headers or something. There are plenty of low level
concepts to think of where one might need this. And even if you can't think
of any reason now, you don't wanna get hit in the face by it and have to
implement such a function for PHP 6.0.1.
For this type of usage, I'd think it'd be relevant to know how many bytes the string will occupy in a given output encoding moreso that what it happens to occupy in the underlying implementation. In the example you cited, string contents will more typically be sent as utf8 rather than the utf16 of php's internal encoding.
$utf8str = unicode_encode($unistr, 'utf8');
header('Content-type: text/html; encoding="utf8"');
header('Content-length: ' . strlen($utf8str));
echo $utf8str;
I'm not saying it's impossible that a legitimate use will come up to know the internal byte-usage of a unicode string, there's certainly no harm in adding such a function (apart from the tired shot-foot argument). I just doubt you (or anyone) will come up such a case anytime soon.
-Sara