Because JSON directly supports the \uxxxx
syntax the first thing that comes into my mind is:
$unicodeChar = '\u1000';
echo json_decode('"'.$unicodeChar.'"');
Another option would be to use mb_convert_encoding()
echo mb_convert_encoding('က', 'UTF-8', 'HTML-ENTITIES');
or make use of the direct mapping between UTF-16BE (big endian) and the Unicode codepoint:
echo mb_convert_encoding("\x10\x00", 'UTF-8', 'UTF-16BE');