How to determine if a string was compressed?

Question

How can I determine whether a string was compressed with gzcompress (aparts from comparing sizes of string before/after calling gzuncompress, or would that be the proper way of doing it) ?

Answer 1

<sub>PRE:
I guess, if you send a request, you can immediately look into $http_response_header to see if the one of the items in the array is a variation of Content-Encoding: gzip. But this is not ideal!
there is a far better method.

Here is HOW TO...</sub>

Check if its GZIP. Like a BOSS!

according to GZIP RFC:

The header of gzip content looks like this

+---+---+---+---+---+---+---+---+---+---+
|ID1|ID2|CM |FLG|     MTIME     |XFL|OS | (more-->)
+---+---+---+---+---+---+---+---+---+---+

the ID1 and ID2 identify the content as GZIP. And CM states that the ZLIB_ENCODING (the compression method) is ZLIB_ENCODING_DEFLATE - which is customarily used by GZIP with all web-servers.

oh! and they have fixed values:

• The value of ID1 is "\x1f"
• The value of ID2 is "\x8b"
• The value of CM is "\x08" (or just 8...)

almost there:

`$is_gzip = 0 === mb_strpos($mystery_string , "\x1f" . "\x8b" . "\x08");`

Working example

<?php
/** @link https://gist.github.com/eladkarako/d8f3addf4e3be92bae96#file-checking_gzip_like_a_boss-php */

date_default_timezone_set("Asia/Jerusalem");

while (ob_get_level() > 0) ob_end_flush();
mb_language("uni");
@mb_internal_encoding('UTF-8');
setlocale(LC_ALL, 'en_US.UTF-8');

header('Time-Zone: Asia/Jerusalem');
header('Charset: UTF-8');
header('Content-Encoding: UTF-8');
header('Content-Type: text/plain; charset=UTF-8');
header('Access-Control-Allow-Origin: *');

function get($url, $cookie = '') {
  $html = @file_get_contents($url, false, stream_context_create([
    'http' => [
      'method' => "GET",
      'header' => implode("\r\n", [''
        , 'Pragma: no-cache'
        , 'Cache-Control: no-cache'
        , 'User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2310.0 Safari/537.36'
        , 'DNT: 1'
        , 'Accept-Language: en-US,en;q=0.8'
        , 'Accept: text/plain'
        , 'X-Forwarded-For: ' . implode(', ', array_unique(array_filter(array_map(function ($item) { return filter_input(INPUT_SERVER, $item, FILTER_SANITIZE_SPECIAL_CHARS); }, ['HTTP_X_FORWARDED_FOR', 'REMOTE_ADDR', 'HTTP_CLIENT_IP', 'SERVER_ADDR', 'REMOTE_ADDR']), function ($item) { return null !== $item; })))
        , 'Referer: http://eladkarako.com'
        , 'Connection: close'
        , 'Cookie: ' . $cookie
        , 'Accept-Encoding: gzip'
      ])
    ]]));

  $is_gzip = 0 === mb_strpos($html, "\x1f" . "\x8b" . "\x08", 0, "US-ASCII");

  return $is_gzip ? zlib_decode($html, ZLIB_ENCODING_DEFLATE) : $html;
}

$html = get('http://www.pogdesign.co.uk/cat/');

echo $html;

What do we see here that is worth mentioning?

• start with initializing the PHP engine to use UTF-8 (since we don't really know if the web-server will return a GZIP content.
• Providing the header Accept-Encoding: gzip, tells the web-sever, it may output a GZIP content.
• Discovering GZIP content (you should use the multi-byte functions with ASCII encoding).
• Finally returning the plain output, is easy using the ZLIB methods.