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Last updated: Thu, 21 Aug 2003

CXV. Zlib Compression Functions


This module enables you to transparently read and write gzip (.gz) compressed files, through versions of most of the filesystem functions which work with gzip-compressed files (and uncompressed files, too, but not with sockets).

Note: Version 4.0.4 introduced a fopen-wrapper for .gz-files, so that you can use a special 'zlib:' URL to access compressed files transparently using the normal f*() file access functions if you prepend the filename or path with a 'zlib:' prefix when calling fopen().

In version 4.3.0, this special prefix has been changed to 'zlib://' to prevent ambiguities with filenames containing ':'.

This feature requires a C runtime library that provides the fopencookie() function. To my current knowledge the GNU libc is the only library that provides this feature.


This module uses the functions of zlib by Jean-loup Gailly and Mark Adler. You have to use a zlib version >= 1.0.9 with this module.


Zlib support in PHP is not enabled by default. You will need to configure PHP --with-zlib[=DIR]

The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.

Note: Builtin support for zlib is available with PHP 4.3.0.

Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

The zlib extension offers the option to transparently compress your pages on-the-fly, if the requesting browser supports this. Therefore there are three options in the configuration file php.ini.

Table 1. Zlib Configuration Options

For further details and definition of the PHP_INI_* constants see ini_set().

Here's a short explanation of the configuration directives.

zlib.output_compression boolean/integer

Whether to transparently compress pages. If this option is set to "On" in php.ini or the Apache configuration, pages are compressed if the browser sends an "Accept-Encoding: gzip" or "deflate" header. "Content-Encoding: gzip" (respectively "deflate") and "Vary: Accept-Encoding" headers are added to the output.

You can use ini_set() to disable this in your script if the headers aren't already sent. If you output a "Content-Type: image/" header the compression is disabled, too (in order to circumvent a Netscape bug). You can reenable it, if you add "ini_set('zlib.output_compression', 'On')" after the header call which added the image content-type.

This option also accepts integer values instead of boolean "On"/"Off", using this you can set the output buffer size (default is 4KB).

Note: output_handler must be empty if this is set 'On' ! Instead you must use zlib.output_handler.

zlib.output_compression_level integer

Compression level used for transparent output compression.

zlib.output_handler string

You cannot specify additional output handlers if zlib.output_compression is activated here. This setting does the same as output_handler but in a different order.

Resource Types

This extension has no resource types defined.

Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

FORCE_GZIP (integer)



This example opens a temporary file and writes a test string to it, then it prints out the content of this file twice.

Example 1. Small Zlib Example


$filename = tempnam ('/tmp', 'zlibtest').'.gz';
print "<html>\n<head></head>\n<body>\n<pre>\n";
$s = "Only a test, test, test, test, test, test, test, test!\n";

// open file for writing with maximum compression
$zp = gzopen($filename, "w9");

// write string to file
gzwrite($zp, $s);

// close file

// open file for reading
$zp = gzopen($filename, "r");

// read 3 char
print gzread($zp, 3);

// output until end of the file and close it.

print "\n";

// open file and print content (the 2nd time).
if (readgzfile($filename) != strlen($s)) {
        echo "Error with zlib functions!";
print "</pre>\n</h1></body>\n</html>\n";

Table of Contents
gzclose -- Close an open gz-file pointer
gzcompress -- Compress a string
gzdeflate -- Deflate a string
gzencode -- Create a gzip compressed string
gzeof -- Test for end-of-file on a gz-file pointer
gzfile -- Read entire gz-file into an array
gzgetc -- Get character from gz-file pointer
gzgets -- Get line from file pointer
gzgetss --  Get line from gz-file pointer and strip HTML tags
gzinflate -- Inflate a deflated string
gzopen -- Open gz-file
gzpassthru --  Output all remaining data on a gz-file pointer
gzputs -- Alias for gzwrite()
gzread -- Binary-safe gz-file read
gzrewind -- Rewind the position of a gz-file pointer
gzseek -- Seek on a gz-file pointer
gztell -- Tell gz-file pointer read/write position
gzuncompress -- Uncompress a deflated string
gzwrite -- Binary-safe gz-file write
readgzfile -- Output a gz-file
zlib_get_coding_type -- Returns the coding type used for output compression

add a note add a note User Contributed Notes
Zlib Compression Functions
chris at mad-teaparty dot com
28-May-2003 07:10
Nice function. I did it the other way round:

function uncompress($srcName, $dstName) {
   $zp = gzopen($srcName, "r");
        $string .= gzread($zp, 4096);

   $fp = fopen($dstName, "w");
   fwrite($fp, $string, strlen($string));

uncompress("./myfile.txt.gz", "./myfile.txt");

A shorter approach would be:

function uncompress($srcName, $dstName) {
  $string = implode("", gzfile($srcName));
  $fp = fopen($dstName, "w");
  fwrite($fp, $string, strlen($string));
devcontact at tech-island dot com
24-Mar-2003 11:47
The method of first reading the source file and then passing its content to the gzip function instead of simply the source and destination filename was a bit confusing for me.

So I have written a simple funtion you can use to compress files in the gzip format (gzip is readable by winzip like .zip files)

function compress($srcName, $dstName)
  $fp = fopen($srcName, "r");
  $data = fread ($fp, filesize($srcName));

  $zp = gzopen($dstName, "w9");
  gzwrite($zp, $data);

// Compress a file
compress("/web/myfile.dat", "/web/myfile.gz");
mlevy at rgj dot com
13-Feb-2003 06:06
If you turn zlib.output_compression_level on, be advised that you shouldn't try to flush() the output in your scripts. PHP will add the gzip header but send the output uncompressed, which plays havoc with Mozilla. IE seems to handle it, though.
monte at ispi dot net
19-Apr-2001 08:02
An alternate way to handle gzip compression is to let the mod_gzip module of apache handle it. This seems to contradict the tutorial on saying that it won't compress php (or any dynamic) output, but mod_gzip as of version works well for me.

Here is an example of an httpd.conf setup:

<IfModule mod_gzip.c>
mod_gzip_on                 Yes
mod_gzip_dechunk            Yes
mod_gzip_minimum_file_size  300
mod_gzip_maximum_file_size  0
mod_gzip_maximum_inmem_size 100000
mod_gzip_keep_workfiles     No
mod_gzip_temp_dir           /tmp
mod_gzip_item_include       file \.html$
mod_gzip_item_include       file \.jsp$
mod_gzip_item_include       file \.php$
mod_gzip_item_include       file \.pl$
mod_gzip_item_include       mime ^text/.*
mod_gzip_item_include       mime ^application/x-httpd-php
mod_gzip_item_include       mime ^httpd/unix-directory$
mod_gzip_item_include       handler ^perl-script$
mod_gzip_item_include       handler ^server-status$
mod_gzip_item_include       handler ^server-info$
mod_gzip_item_exclude       mime ^image/.*

This will automatically compress all output of your files with the .php extention or the x-httpd-php mime type. Be sure to have dechunk set to Yes.

 Last updated: Thu, 21 Aug 2003
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