$s = "c0 0b 12 db a4 d6 e4 7e";
$hex = sscanf($s, "%02x %02x %02x %02x %02x %02x %02x %02x");
$hex = array_reverse($hex);
$bin = implode('', array_map('chr', $hex));
$array = unpack("dnum", $bin);
echo $array['num'];