PHP using gzip to compress the method of transmitting js and css files

<?php
  /**
    * Complete call example:
    * 1. ?t=j&b=public&fs=,function
    *
    * This example calls public/jslib/ and public/ in the root directory of the website
    *
    * 2. ?t=j&fs=,function
    *
    * This example calls jslib/ and
    *
    * 3. ?t=c&b=&fs=common,index
    *
    * This example calls public/css/ and public/css/ in the root directory of the website
    *
    * 4. ?t=c&fs=
    * This example calls css/ in the root directory of the website
    *
    * Note: Use multiple file names to separate them; only one file name should not be included at the end.
    * Use, multiple separated files will be compressed into one file and passed to the browser at one time.
    **/
  $is_bad_request=false;
  $cache = true;
  $doc_root_uri=$_SERVER['DOCUMENT_ROOT'].'/';
  $cachedir = $doc_root_uri . 'public/cache';
  //File type, j is js, c is css  $type=isset($_GET['t'])?($_GET['t']=='j'||$_GET['t']=='c'?$_GET['t']:''):'';
  //The base directory for storing js and css files, for example: ?b= represents the /public/js folder, and the starting point is the website root directory  //Base directory parameters are not required. If there is a base directory, then this base directory will be appended before the file name  $base =isset($_GET['b'])?($doc_root_uri.str_replace('.','/',$_GET['b'])):$doc_root_uri;
  //File name list, file name without suffix name. For example, the base directory is  //The format of the file name is: base directory (if any) + file package name + file name  //For example: type is j,  //   file name  // If there is a base path and it is public,  // Then the converted file name is /public/public/js/  // If there is no base path  // Then the converted file name is /public/js/  // Use multiple file names to separate  $fs=isset($_GET['fs'])?str_replace('.','/',$_GET['fs']):'';
  $fs=str_replace(',','.'.($type=='j'?'js,':'css,'),$fs);
  $fs=$fs.($type=='j'?'.js':'.css');
  if($type==''||$fs==''){$is_bad_request=true;}
  //die($base);
  if($is_bad_request){header ("HTTP/1.0 503 Not Implemented");}
  $file_type=$type=='j'?'javascript':'css';
  $elements = explode(',',preg_replace('/([^?]*).*/', '\1', $fs));
  // Determine last modification date of the files
  $lastmodified = 0;
  while (list(,$element) = each($elements)) {
    $path =$base . '/' . $element;
    if (($type == 'j' && substr($path, -3) != '.js') || 
      ($type == 'c' && substr($path, -4) != '.css')) {
      header ("HTTP/1.0 403 Forbidden");
      exit;  
    }
    if (substr($path, 0, strlen($base)) != $base || !file_exists($path)) {
      header ("HTTP/1.0 404 Not Found");
      exit;
    }
    $lastmodified = max($lastmodified, filemtime($path));
  }
  // Send Etag hash
  $hash = $lastmodified . '-' . md5($fs);
  header ("Etag: \"" . $hash . "\"");
  if (isset($_SERVER['HTTP_IF_NONE_MATCH']) && 
    stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) == '"' . $hash . '"') 
  {
    // Return visit and no modifications, so do not send anything
    header ("HTTP/1.0 304 Not Modified");
    header ("Content-Type: text/" . $file_type);
    header ('Content-Length: 0');
  } 
  else
  {
    // First time visit or files were modified
    if ($cache) 
    {
      // Determine supported compression method
      $gzip = strstr($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip');
      $deflate = strstr($_SERVER['HTTP_ACCEPT_ENCODING'], 'deflate');
      // Determine used compression method
      $encoding = $gzip ? 'gzip' : ($deflate ? 'deflate' : 'none');
      // Check for buggy versions of Internet Explorer
      if (!strstr($_SERVER['HTTP_USER_AGENT'], 'Opera') && 
        preg_match('/^Mozilla\/4\.0 \(compatible; MSIE ([0-9]\.[0-9])/i', $_SERVER['HTTP_USER_AGENT'], $matches)) {
        $version = floatval($matches[1]);
        if ($version < 6)
          $encoding = 'none';
        if ($version == 6 && !strstr($_SERVER['HTTP_USER_AGENT'], 'EV1')) 
          $encoding = 'none';
      }
      // Try the cache first to see if the combined files were already generated
      $cachefile = 'cache-' . $hash . '.' . $file_type . ($encoding != 'none' ? '.' . $encoding : '');
      if (file_exists($cachedir . '/' . $cachefile)) {
        if ($fp = fopen($cachedir . '/' . $cachefile, 'rb')) {
          if ($encoding != 'none') {
            header ("Content-Encoding: " . $encoding);
          }
          header ("Content-Type: text/" . $file_type);
          header ("Content-Length: " . filesize($cachedir . '/' . $cachefile));
          fpassthru($fp);
          fclose($fp);
          exit;
        }
      }
    }
    // Get contents of the files
    $contents = '';
    reset($elements);
    while (list(,$element) = each($elements)) {
      $path = $base . '/' . $element;
      $contents .= "\n\n" . file_get_contents($path);
    }
    // Send Content-Type
    header ("Content-Type: text/" . $file_type);
    if (isset($encoding) && $encoding != 'none') 
    {
      // Send compressed contents
      $contents = gzencode($contents, 9, $gzip ? FORCE_GZIP : FORCE_DEFLATE);
      header ("Content-Encoding: " . $encoding);
      header ('Content-Length: ' . strlen($contents));
      echo $contents;
    } 
    else
    {
      // Send regular contents
      header ('Content-Length: ' . strlen($contents));
      echo $contents;
    }
    // Store cache
    if ($cache) {
      if ($fp = fopen($cachedir . '/' . $cachefile, 'wb')) {
        fwrite($fp, $contents);
        fclose($fp);
      }
    }
  }