Some of code examples here serialize data length as 8 bytes, e.g.

$packet = "ZBXD\1" . pack('P', strlen($data)) . $data;

according to the old protocol specification:

<DATALEN> - data length (8 bytes). 1 will be formatted as 01/00/00/00/00/00/00/00 (eight bytes, 64 bit number in little-endian format)

Nowadays data length is supposed to be 4 bytes and another 4 bytes are "reserved":

<DATALEN> - data length (4 bytes). 1 will be formatted as 01/00/00/00 (four bytes, 32 bit number in little-endian format).
<RESERVED> - reserved for protocol extensions (4 bytes).

P.S. It also says that

When compression is enabled (0x02 flag) the <RESERVED> bytes contains uncompressed data size, 32 bit number in little-endian format.

...but does not say what <RESERVED> bytes should be when compression is not enabled.

P.P.S. Use of <code> seems inappropriate to list parts of protocol header and looks a bit ugly. A list or a table would look better.