Question

I have just bought a GPS Tracker, it can send SMS to cellphone just fine. It also supports reporting to a server via GPRS.

I have setup the device to contact my own server on port 8123, it’s a FreeBSD server and i have checked that i recieve packets on that port.

I successfully have setup a listener server written in PHP, and i can receive data from the device. But how do i convert the partial HEX data to something usefull (ASCII)?

Example data string:

$$^@T^@E Y'^WÿU210104.000,A,5534.4079,N,01146.2510,E,0.00,,170411,,*10|1.0|72|0000á

Unfortunately i don’t know how i can copy-paste the HEX parts

Now how do i get the ID part out? I have tried echo hexdec(mb_substr($data, 4, 7));

The data is following this protocol

From the document:

Command format of GPRS packets are as follows:

From server to tracker:
@@\r\n 

From tracker to server: 
$$\r\n

Note: 
Do NOT input ‘’ when writing a command. 
All multi-byte data complies with the following sequence: High byte prior to low byte. 
The size of a GPRS packet (including data) is about 100 bytes

Item        Specification
@@          2 bytes. It means the header of packet from server to tracker. 
            It is in ASCII code (Hex code: 0x40)

$$          2 bytes. It is the header of packet from tracker to server.
            It is in ASCII code (Hex code: 0x24)

L           2 bytes. It means the length of the whole packet including 
            the header and ending character and it is in hex code

ID          7 bytes, ID must be digit and not over 14 digits, the unused byte 
            will be stuffed by ‘f’ or ‘0xff’. It is in the format of hex code.
            For example, if ID is 13612345678, then it will be shown as 
            follows: 0x13, 0x61, 0x23, 0x45, 0x67, 0x8f, 0xff.
            If all 7 bytes are 0xff, it is a broadcasting command. ID is in hex code

command     2 bytes. The command code is in hex code. Please refer to the 
            command list below.

data        Min 0 byte and max 100 bytes. See Annex 1 for description of ‘data’.

checksum    2 bytes. It indicates CRC-CCITT (default is 0xffff) checksum of
            all data (not including CRC itself and the ending character).
            It is in hex code. 
            For example: 24 24 00 11 13 61 23 45 67 8f ff 50 00 05 d8 0d 0a
            0x05d8 = CRC-CCITT (24 24 00 11 13 61 23 45 67 8f ff 50 00)

\r\n        2 bytes. It is the ending character and in hex code 
            (0x0d,0x0a in hex code)

Update

With the answer from Anomie, i was able to piece this together

$arr = unpack('H4length/H14id/H4cmd/H4crc/H4end', mb_substr($data, 2, 11) . mb_substr($data, -4));

var_dump($arr);

This will out put something like

array(5) {
  ["length"]=>
  string(4) "0054"
  ["id"]=>
  string(14) "004512345678ff"
  ["cmd"]=>
  string(4) "9955"
  ["crc"]=>
  string(4) "c97e"
  ["end"]=>
  string(4) "0d0a"
}

Answer 1

It sounds like you are needing to convert binary data to integers or strings. The most straightforward way is to use unpack.

For example, to extract the length you already know you can use

$length_bin = substr($string, 2, 2);

To convert it to an integer, you can use something like

$length = unpack('v', $length_bin); $length = $length[1];

The 'v' code will work for the length and the checksum; if you have a number stored as 4 bytes use 'V', and for the ID you can use 'H*' to get it as a string of hex digits. Other codes are listed in the documentation.

---

A somewhat less straightforward way is to do the bit manipulation manually, after using unpack with C* to get an array of all the byte values. For example,

$bytes = unpack('C*', $length_bin);
$length = ($bytes[0] << 8) | $bytes[1];