[Ghost]

даже лучше так

function makeChar($numArr){
	$num=0xd090+ord($numArr[1])-192;
	if($num>0xd0BF) $num=$num-0xd0BF+0xd17F;

	return chr($num>>8).chr($num & 0x00FF);
}

echo chr(0xEF).chr(0xBB).chr(0xBF);
echo preg_replace_callback("/([А-Яа-я])/","makeChar",$str);