use PHP to unpack, process and pack array - php

Question

I read bytes from a file and process them. Afterwards I would like to save the packed bytes.
What is the recommended+generic way to convert an array with mixed objects/types to a byte string? In my case: array with int and string, pack types a,C,x.
A simplified example:
// $bytes = fread($handle, 100);
$bytes = "437XYZ25.011001DBEFORE ....";
$unpackString = "a3CPN/x8spare/CDSC/x4spare/a32OPT";
$unpacked = unpack($unpackString, $bytes);
var_dump($unpacked);
/*
array(3) {
["CPN"]=> string(3) "437"
["DSC"]=> int(49)
["OPT"]=> string(32) "BEFORE "
}
*/
// example of processing
$unpacked["DSC"] = 12;
$unpacked["OPT"] = "AFTER ";
// pack + write the result
// $packString = "a3x8Cx4a32";
$packTypes = ["a3","x8","C","x4","a32"];
$packFields = [ $unpacked["CPN"], null, $unpacked["DSC"], null, $unpacked["OPT"] ];
// ...
update: in the simplified example I have replaced $packString with $packTypes and $packFields to make sure it is clear what content belongs where and with what type.

Answer 1

I suppose what you're looking for is a way to call pack, which accepts arguments with an associative array as you have in your example. For this, we can use call_user_func_array which calls a function by its name and provides its arguments from a given array.
$bytes = "437XYZ25.011001DBEFORE ....";
$unpackString = "a3CPN/x8spare/CDSC/x4spare/a32OPT";
$unpacked = unpack($unpackString, $bytes);
// example of processing
$unpacked["DSC"] = 12;
$unpacked["OPT"] = "AFTER ";
// pack + write the result
$packTypes = ["a3", "x8", "C", "x4", "a32"];
$packFields = [$unpacked["CPN"], null, $unpacked["DSC"], null, $unpacked["OPT"]];
$packString = "";
$packArguments = [];
for ($i = 0; $i < count($packTypes); $i++){
$packString .= $packTypes[$i];
if ($packFields[$i] !== null){
// the null bytes don't use an argument
$packArguments[] = $packFields[$i];
}
}
// put packString as the first argument
array_unshift($packArguments, $packString);
$output = call_user_func_array("pack", $packArguments);
And $output would then be:
00000000 34 33 37 00 00 00 00 00 00 00 00 0c 00 00 00 00 |437.............|
00000010 41 46 54 45 52 20 20 20 20 20 20 20 20 20 20 20 |AFTER |
00000020 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
00000030