php gzdecode() replacement/extension

gzdecodeM.php

<?php
/** function 'gzdecodeM'
* @param data string .GZ data
* @param gz array (optional) Returned array of file sections properties
* @param error string (optional) Returned error (if any)
* @param maxlength int (optional) Max length of uncompressed data
* @param sects string (optional) Sections to uncompress, e.g. '0-3, 5, 8-p100000, 100-' (where: '0' - section number, 'p0' - file .gz position in Bytes, 'x-' - from section x to the end of file)
*      Note: sections are extracted once and in the order they are in the .gz file, not how they are listed
* @return Uncompressed data (if OK)
*/
function gzdecodeM($data, &$gz, &$error='', $maxlength=null, $sects='') {

  $section = 0;
  $member = 0;//gzip member position in $data
  $len0 = strlen($data);
  
  if ($sects) {
    $sects=explode(',', $sects);
    foreach ($sects as &$sect) {
      $sect=explode('-', $sect);
      $sect[0]=trim($sect[0]);
      if (strtolower($sect[0][0])==='p') {$sect[2]=substr($sect[0], 1); $sect[0]='p';}
      if(isset($sect[1])) {
        $sect[1]=trim($sect[1]);
        if (strtolower($sect[1][0])==='p') {$sect[3]=substr($sect[1], 1); $sect[1]='p';}
      }
    }
    unset($sect);
  }
  
  do {
    $gz[$section]=array();
    
    $len = strlen($data)-$member;
    if ($len < 18 || strcmp(substr($data,$member+0,2),"\x1f\x8b")) {
      $error = "Not in GZIP format.";
      return null;  // Not GZIP format (See RFC 1952)
      // http://www.faqs.org/rfcs/rfc1952.html
    }
    $method = $gz[$section]['method'] = ord(substr($data,$member+2,1));  // Compression method
    switch ($method) {
      case 8:
        // Currently the only supported compression method:
        break;
      default:
        $error = "Unknown compression method.";
        return false;
    }

    $flags  = $gz[$section]['flags'] = ord(substr($data,$member+3,1));  // Flags
    if ($flags & 31 != $flags) {
      $error = "Reserved bits not allowed.";
      return null;
    }
    // NOTE: $mtime may be negative (PHP integer limitations)
    $gz[$section]['mtime'] = unpack("V", substr($data,$member+4,4));
    $gz[$section]['mtime'] = $gz[$section]['mtime'][1];
    //$xfl   = substr($data,$member+8,1); // @TODO - for future use
    $gz[$section]['os']    = ord(substr($data,$member+9,1));
    $headerlen = 10;
    if ($flags & 4) {
      // 2-byte length prefixed EXTRA data in header
      if ($len - $headerlen - 2 < 8) {
        return false;  // invalid
      }
      $extralen = unpack("v",substr($data,$member+10,2));
      $extralen = $extralen[1];
      if ($len - $headerlen - 2 - $extralen < 8) {
        return false;  // invalid
      }
      $gz[$section]['extra'] = substr($data,$member+12,$extralen);
      $headerlen += 2 + $extralen;
    }
    $filenamelen = 0;
    $gz[$section]['filename'] = "";
    if ($flags & 8) {
      // C-style string
      if ($len - $headerlen - 1 < 8) {
        return false; // invalid
      }
      $filenamestart=$headerlen+( isset($extralen) ? 2 : 0 )+$extralen;
      $filenamelen = strpos(substr($data,$member+$filenamestart),chr(0));
      if ($filenamelen === false || $len - $filenamestart - $filenamelen - 1 < 8) {
        return false; // invalid
      }
      $gz[$section]['filename'] = substr($data,$member+$filenamestart,$filenamelen);
      $headerlen =$filenamestart + $filenamelen + 1;
    }
    $commentlen = 0;
    $gz[$section]['comment'] = "";
    if ($flags & 16) {
      // C-style string COMMENT data in header
      if ($len - $headerlen - 1 < 8) {
        return false;    // invalid
      }
      $commentlen = strpos(substr($data,$member+$headerlen),chr(0));
      if ($commentlen === false || $len - $headerlen - $commentlen - 1 < 8) {
        return false;    // Invalid header format
      }
      $gz[$section]['comment'] = substr($data,$member+$headerlen,$commentlen);
      $headerlen += $commentlen + 1;
    }
    $headercrc = "";
    if ($flags & 2) {
      // 2-bytes (lowest order) of CRC32 on header present
      if ($len - $headerlen - 2 < 8) {
        return false;    // invalid
      }
      $calccrc = crc32(substr($data,$member+0,$headerlen)) & 0xffff;
      $headercrc = unpack("v", substr($data,$member+$headerlen,2));
      $headercrc = $headercrc[1];
      if ($headercrc != $calccrc) {
        $error = "Header checksum failed.";
        return false;    // Bad header CRC
      }
      $headerlen += 2;
    }
    
    
    
    
    
    // compressed blocks extraction...
    $restlen = $len-$headerlen-8;
    if ($restlen < 1) {
      // IMPLEMENTATION BUG!
      return null;
    }
    $rest = $comprssd = substr($data,$member+$headerlen,$restlen);
    $next = 8;
    $uncomprssd = $members = false;

    //Searching for next member
    while ( ($next !== false) && ($uncomprssd === false) ) {
    
      // Searching for next gzip-file-members - see https://stackoverflow.com/questions/13112604/find-gzip-start-and-end
      $next=strpos($rest, "\x1f\x8b", $next+1);
      if ($next > $restlen) $next = false;

      if ($next!==false) {
        $flags  = ord(substr($rest,$next+3,1));  // Flags
        if ($flags & 31 == $flags) {
          $comprssd = substr($rest, 0, $next-8);
          $members = true;
        }
      } else {
        if ($members) $comprssd = $rest;
      }

      if ($next==8) {
        $uncomprssd = "";  // zero-byte body content is allowed
        $gz[$section]['size_packed'] = 0;
      } else {
      
        // Note: Need to decompress even it's not $this_sect, as a test - to be sure this is proper compressed section
        $uncomprssd = gzinflate($comprssd,$maxlength);
        $gz[$section]['size_packed'] = strlen($comprssd);
      }

    }

    if ($uncomprssd===false) {
      $error = "Compressed data failed or decompression error.";
      return false;  //error
    }

    if ($next === false) $next = $restlen+8;
    
    // GZIP FOOTER
    $datacrc = unpack("V",substr($data, $member+$headerlen+$next-8, 4));
    $datacrc = sprintf('%u',$datacrc[1] & 0xFFFFFFFF);
    $gz[$section]['CRC32'] = strtoupper(str_pad(dechex($datacrc), 8, '0', STR_PAD_LEFT));
    $isize = unpack("V",substr($data, $member+$headerlen+$next-4, 4));
    $isize = $gz[$section]['size'] = $isize[1];
    
    $this_sect = false;
    $member0 = $member;
    $member += $next + $headerlen;

    
    if ($sects)
    foreach ($sects as $sect) if (!$this_sect) {
      $this_sect =
        (($section==$sect[0])&&(!isset($sect[1])))
        || ( (  // testing initial range
            ($section>=$sect[0])
            || (($sect[0]==='p')&&($sect[2]<=$member0))
          )
          && (  // testing final range
            ($sect[1]==='')  // ...to the end of data
            || ($section<=$sect[1])
            || (($sect[1]==='p')&&($member-1<=$sect[3]))
          )
        );
    }
    
    if (($this_sect)||(!$sects)) {
      $wholedata.= $uncomprssd;
      
      // Verifiy CRC32
      $crc   = sprintf("%u",crc32($uncomprssd));
      $gz[$section]['CRC32_OK'] = ( ($crcOK = $crc == $datacrc) ? true : false );
      $lenOK = $isize == strlen($uncomprssd);

      if (!$lenOK || !$crcOK) {
        $error = ( $lenOK ? '' : 'Length check FAILED. ') . ( $crcOK ? '' : 'Checksum FAILED.');

        return false;
      }
    } else $gz[$section]['not_selected'] = 1;
    $section++;
  } while ($member < $len0);
  

  
  return $wholedata;
}
// expanded from http://php.net/manual/en/function.gzdecode.php#82930
?>

Searching for gzip headers is neither reliable nor necessary. You can instead use inflate_init(), inflate_add(), and inflate_get_read_len(). The latter will tell you how many bytes of the compressed data was consumed to inflate the last gzip member. You start the next inflate there.