PHP extract data from binary file with structure - php

Question

I have i file with structure in image. I want to extract data to array from that:
function get_data($file, $number)
{
if(!$fp = fopen ($file, 'rb')) return 0;
$fsize = filesize($file);
if(!$data = fread ($fp, $fsize)) return 0;
$data_format=
'#100/'.
'smember_id/'.
'cmember_name_length/'.
'a' . $member_name_length . 'member_name/'.
'C100other_data/';
$data = unpack ($data_format, $data);
fclose($file);
return $data;
}
How can I get the $member_name_length from the file? I want to create a function that if user input the $number, it returns a array of $number(th) data.
Thank you.

Answer 1

Since you have a variable-length data blocks, you can read them only sequentially, so in order to read n-th block, you need to read all n first blocks:
function readDataBlock($f) {
$data = unpack('nmember_id', fread ($f, 2)); // I assume member_id is n, not s
if ($data['member_id'] == 0xFFFF) {
throw new \Exception('End of file');
}
$data = array_merge($data, unpack('Cmember_name_length', fread ($f, 1))); //again, it must be C, not c, as I can't imagine negative length.
$data = array_merge($data, unpack('a*member_name', fread ($f, $data['member_name_length']))); // be sure you understand how a differs from A
return array_merge($data, unpack('C100other_data', fread ($f, 100))); // are you sure C100 is what you want here?
}
function get_data($file, $number)
{
if(!$fp = fopen ($file, 'rb')) return 0;
fread ($fp, 100); //skip header
for($n = 0; $n <= $number; $n++) {
$data = readDataBlock($fp); // read single member
}
fclose($fp);
return $data; //return the n-th member
}
If the file is small enough to fit into memory, it might be better to read it once and return n-th member from memory:
$data = [];
while(true) {
try {
$data[] = readDataBlock($fp);
} catch(\Exception $e) {
break;
}
}
function get_data(&$data, $number)
{
return $data[$number];
}