// assume we have 52 bits of data, little endian.
$bytes = "\x12\x34\x56\x78\x9a\xbc\x0d\x00";
//                                  ^   ^^ 12 bits of padding.
 
$parts = unpack('V2', $bytes);
 
$theNumber = $parts[1] + pow(2.0, 32) * $parts[2];  // <-- this is precise.
$theNumber /= pow(2.0, 52);                         // <-- this is precise.