Quick Table of Contents

  1. Introduction
  2. Test your server installation
  3. Enabling output bufferring
  4. Test that it works
  5. Results
  6. Further tweaks
  7. Further reading

Using gzip compression on PHP pages

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1. Introduction

One of the frequently overlooked functions of servers with PHP of versions 4.04 and above is the ability to compress output. By compressing output, you can speed up page load times and reduce bandwidth.

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2. Test your server installation

First, check that your server supports gzip compression. If your site has been hosted by us, this would be enabled by default.

Create a file, phpinfo.php and upload to your webserver to test: 

    <?php
      phpinfo();
    ?>

Look for the values for HTTP_ACCEPT_ENCODING -- you should hopefully find the values gzip,deflate meaning compression is supported.

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3. Enabling output bufferring

Once you've determinted that your server supports gzip, you will need to turn it on in your PHP code.

Turn on output bufferring with ob_start

ob_start -- Turn on output buffering.

bool ob_start ( [callback output_callback [, int chunk_size [, bool erase]]] )

While output bufferring is active no output is sent from the script, it is stored in an internal buffer until ob_end_flush is called to output the buffer to the browser at the end of the request.

Calling ob_start with ob_gzhandler

Now, add the callback function ob_gzhandler which will determine what type of content encoding the browser will accept ("gzip", "deflate" or none at all).

Ensure that the following line appears at the top of the pages that you want compressed:

      <?php
        ob_start("ob_gzhandler");
      ?>

Flush the output to the browser with ob_end_flush

At the end of the pages that you've set to compress, ensure that you include the following line of code:

      <?php
        ob_end_flush();
      ?>
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4. Test that it works

You can test if your browser supports gzip'ed content, and if your site is using gzip here.

You can also use wget to test if your pages are now gzip'ed: 

wget --header="Accept-Encoding: gzip,*" -S http://your.site.com
--11:28:28--  http://www.example.com/
           => `index.html'
Resolving www.example.com... xxx.xxx.xxx.xxx
Connecting to www.example.com|xxx.xxx.xxx.xxx|:80... connected.
HTTP request sent, awaiting response...
  HTTP/1.1 200 OK
  Date: Fri, 09 Dec 2005 00:29:04 GMT
  Server: Apache/1.3.33 (Unix) mod_auth_passthrough/1.8 mod_log_bytes/1.2 mod_bwlimited/1.4 PHP/4.3.11 FrontPage/5.0.2.2635 mod_ssl/2.8.22 OpenSSL/0.9.7a
  X-Powered-By: PHP/4.3.11
  Content-Encoding: gzip
  Vary: Accept-Encoding
  Connection: close
  Content-Type: text/html
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5. Results

The following results were obtained from the same site:

without gzip: 121.52 KB

with gzip: 26.49 KB

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6. Further tweaks

The compression level is set to 6 by default. A higher compression level means more load on the server's CPU. You might want to experiment with the levels to find a value that will give you a good compression for an acceptable amount of load on the server.

To change the compression level, simply insert the following line before ob_start("ob_gzhandler"): 

ini_set('zlib.output_compression_level', 4);

An ideal level would be between 1 and 6.

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7. Further reading

Links