mb_convert_encoding() with UTF-16 input in PHP > 8.1

I'm updating a PHP app which imports CSV encoded in UTF-16 (from Google Keyword Planner) and the values are converted to UTF-8.

Until PHP 8 it's working as expected, but from PHP 8.1 there is a ? added to the values after the conversion from UTF-16 to UTF-8:

var_dump(mb_convert_encoding("\0008\0008\0000\000", "UTF-8", "UTF-16"));

// Output with PHP 8.1.3 - 8.1.13, 8.2.0:
// string(4) "880?"

// Output with PHP 7.4.32, 8.0.8 - 8.0.26:
// string(3) "880"

Your source equals to "\x00\x38\x00\x38\x00\x30\x00", which is 7 bytes and as such an invalid length for UTF-16, which always needs 2 or 4 bytes per character.

• You're lucky enough PHP7 did silently accept the first 6 bytes and drop the 7th,
• while PHP8 now produces a more correct output as per UTF-16 LE and wants to tell you that there is an imcomplete 4th character, because there's only 1 byte for it.

Solution: provide proper input. Maybe it's also because you misunderstood the octal notation and would see it much better without mixing notation and literals altogether: