In #17863 a binary check is introduced like
preg_match('/[^\x09-\x0d\x20-\xff]/', $value)

This is not specific, in the sense that some invalid utf8 won't be recognized as binary.

The recognition is nontrivial iterating a byte sequence, but PHP has some simple one-liners to do it like
$isUTF8 = preg_match('//u', $string);