This is a response to last week's question on how to extract bits out 
of a string of bytes.

Assume $packet is a string of 8 bytes.

To extract the second byte:
$second = unpack ('C', substr($packet, 1, 1));

To extract the last four bytes
$last_four = unpack ('N', substr($packet, 4, 4));

To extract the first two:
$first_two = unpack ('n', substr($packet, 0, 2));

To extract the first four bits from the second byte:
$first_four_bits = int (unpack ('C', substr($packet, 1, 1)) / 16);

To extract the second four bits from the second byte:
$second_four_bits = (unpack ('C', substr($packet, 1, 1)) % 16);

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