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PHP : Function Reference : Output Control Functions : ob_gzhandler


ob_start callback function to gzip output buffer (PHP 4 >= 4.0.4, PHP 5)
string ob_gzhandler ( string buffer, int mode )

Example 1689. ob_gzhandler() example



<p>This should be a compressed page.</p>

Code Examples / Notes » ob_gzhandler


When writing scripts for distribution, I would usually "null" out the following deprecated superglobals so that users who uses the script will not be able to use them.
$HTTP_GET_VARS = null;
However, when using ob_start('ob_gzhandler'), one of those superglobals somehow disable this function.
Found out that it was $HTTP_SERVER_VARS that's causing the problems.
$HTTP_GET_VARS = null;
* Causing the trouble
I don't know why it does that, but I just want to point that out.

09-apr-2003 06:47

When using ob_start("ob_gzhandler"), be aware that the output buffer has to be flushed in order to invoke the ob_gzhandler callback function.
This may not always happen. For example, if you use ob_get_contents() to copy the output buffer to a string for further manipulation and then silently discard the buffer with ob_end_clean(), the output buffer is never "flushed" and as a result, the ob_gzhandler callback function is never invoked. And your pages won't be compressed.
This will be the case, for example, if you use the PHP Fusebox architecture/framework.


To pass the second argument to ob_gzhandler() which specifies what level of compression should be used (which I assume is 1-9 as with the gzip binary, with 9 using the most processor and time to do, 4 is the standard IIRC) you must call ob_start() like so:
- Davey


To get this to work properly, you have to compile PHP with "--with-zlib". If you don't, you won't get any errors, it just won't actually compress anything. This is a phenomenal thing. For just a small bit of processor time, you can DRASTICALLY reduce the bandwidth requirement of your scripts. There are very few circumstances in which this should not be used.


Someone previously mentioned that MSIE doesn't cache gzipped content. This is false. He misread the source of information. In fact, IE will cache gzipped content no matter what. Here is the quote from the mailing list:
The reason the COMPRESSED responses are, in fact,
always getting cached no matter what "Vary:" field name
is present is just as I suspected... it is because MSIE
decides it MUST cache responses that arrive with
"Content-Encoding: gzip" because it MUST have a
disk ( cache ) file to work with in order to do the


So why doesn't the web server just do this by default?  
This works for me if I do:
ini_set('zlib.output_compression_level', 3);  ob_start("ob_gzhandler");
or even just:
I did the level 3 compression, I think the default was 6 and I didn't want to put too much load on the server.  For a 895k file (my largest) the compression levels were:
1 = 189k
3 = 178k
4 = 163k
6 = 156k    (I believe 6 is the default if you leave out the ini_set)
9 = 155k
I use to check the sizes.
FYI: This works with dynamic files in Movable Type 3.x (I'm testing it in 3.2) I've got the above command in the first line of my mtview.php file.
More info here:
On several sites I've read the some browsers don't like compressed CSS.


So much's a lighter/faster one:
if ( $do_gzip_compress )
$gzip_size = ob_get_length();
$gzip_contents = ob_get_clean(); // PHP < 4.3 use ob_get_contents() + ob_end_clean()

echo "\x1f\x8b\x08\x00\x00\x00\x00\x00",
substr(gzcompress($gzip_contents, 9), 0, - 4), // substr -4 isn't needed
pack('V', crc32($gzip_contents)), // crc32 and
pack('V', $gzip_size);   // size are ignored by all the browsers i have tested
I wouldn't recommande using level 9, waste of CPU, something between 1 and 6 would be more resonable
I have tested without the substr/crc/size and it worked flawlessly in IE5/5.5/6, Opera7.11/7.21, IE5.5 MAC


RFC 2616 suggests that a more correct way to compress documents on the fly is to use gzip transfer encoding rather than gzip content encoding.    However, it's perhaps not advisable to use this since client support for it is extremely limited.
The difference is most evident when you come to save a file from your browser.  If it's gzip content encoded then the browser should (and may well) save it gzipped.  If it's gzip transfer encoded then the browser should uncompress it first.


Problem revelate by
cause real problem when you have a global call
to ob_gzhandler , for example in an include configuration
file , and you want send a non gzip content , for any reason ...
To resolve it ,
put this after your global call to ob_gzhandler
-->header("Content-Encoding: none");
If you try to overload the header before the the ob_gzhandler call
or during the buffering output , it will not work .
( probably erase at the call of ob_gzhandler  and not allow during
buffering ).
Hope this help


Please be aware that, as other users have already mentioned, the compression will fail if there are characters before the php start tag "< ?".
This is also the case when saving files in UTF-8 format with editors such as Ultraedit. Make sure you save the file using the defined option UTF-8 NO BOM or delete the BOM otherwise the two UTF BOM characters will be written at the start of the file.


One of the earlier posts says caching of gzipped files is broken in all versions of MSIE. The following blog entry appears to declare that this has been fixed in IE7, but I haven't verified this.

24-may-2005 11:27

No matter the circumstances, you MUST NOT check the user agent string to find out if gzip compression is enabled. HTTP defines the Accept-Encoding header for exactly this purpose and you MUST check it before enabling compression.


My PHP version is 5.2.1 For windows.
If browser doesn't support compressed ,
ob_start('ob_gzhandler') returns the original string,
but $str = ob_gzhandler ( $buffer, 5 ) returns false;
/* 1.php */
ob_start('ob_gzhandler') ;
echo 'This is a string.';
/* 2.php */
header("Content-Encoding: gzip");
$buffer = 'This is a string.';
$str = ob_gzhandler ( $buffer, 5 ) ;
   echo 'ob_gzhandler() returns false.';
   echo $str;
/* 3.php */
echo file_get_contents('');
echo file_get_contents('');
This is a string.ob_gzhandler() returns false.


It may seem obvious but if you use ob_start("ob_gzhandler"),  make sure no content is unintentionally echo'ed before : this could be blank lines before '<?php' or after '?>' tags in included files or even errors.
Otherwise, only a part of the content (the content sent after ob_start()) will be compressed, which will confuse the client.
Setting compression using zlib.output_compression either in php.ini or in Apache configuration file (with directive php_flag) is safer in this regard.


It is also possible to use ob_gzhandler to compress css and javascript files, however some browsers such as firefox expect content type text/css on css files.
To get around this send a content type header:
.... your css content ...
header("Content-Type: text/css");
header("Content-Length: ".ob_get_length());


In the set_error_handler notes, there is a technique described for capturing all errors (even parse errors) before they are displayed the user, using a special error handler and output handler. If this output handler detects a fatal error in the output buffer, it's captured and dealt with before it can be displayed to the user. If no error was detected, then output buffer is displayed verbatim (i.e. without being compressed).
If you are using this method, you can still take advantage of ob_gzhandler's compression. However, you MUST specify a mode argument (I'm using 4.2.2 on RedHat9). The mode value affects which headers are automatically added (Content-Encoding, etc). A value of '5' worked for me. '0' or discarding the argument produces a blank screen under Mozilla.
function my_output_handler(&$buffer) {
 // Detect errors in the output
 if(ereg("(error</b>:)(.+) in <b>(.+)</b> on line <b>(.+)</b>", $buffer, $regs)) {
   my_error_handler(E_ERROR, $regs[2], $regs[3], $regs[4]);
   // ...
   // ... Insert your error handling here ...
   // ...
   return 'An internal error occurred.';
 } else {
   // The page rendered without any errors, so compress
   // and output.
   return ob_gzhandler($buffer, 5);


If you want to suppress errors (such as for cases when headers are already sent) and you don't want to write or patch your error_handler, you need to create an intermediate callback:
function ob_gz_handler_no_errors($buffer)


if you want to send an output to the browser (which accepts gzip), and you haven't set the buffering callback ob_start("ob_gzhandler"), you may use the gzencode() function.
header("Content-Encoding: gzip");
echo gzencode("some output", 9, FORCE_GZIP);


if you get errors like:
"output handler 'ob_gzhandler' cannot be used twice"
if you are using
check for your php.ini file, it should look like this:
output_buffering = Off  ; delete the 4096k value
output_handler =
zlib.output_compression = Off


if you call ob_end_clean() after ob_start("ob_gzhandler"), the "Content-Encoding: gzip" header will still get sent (assuming the browser supports the encoding).  if you don't call ob_start() again with the ob_gzhandler callback function, the output will not be compressed, but the header will say it is.  this causes mozilla (as of build 2002032808) to display a blank page.


If you are trying to use the ob_gzhandler in a MSHTA (Microsoft HTML Application) you will find that your pages does not show up. the MSHTA-browser probably says it can handle gzip, but it just won't work (IE:6, PHP:4.1.2)


IE 6+ appears to validate the CRC and disgard invalid gzip content FYI...


I wrote a simple script that I use on my site. Bandwidth consumption dropped dramatically. Pages shrunk 3-5 times.
The script simulates the results of using this function directly but by using regular output buffering with ob_start. So you get to output gzip and play with it before you do.
You can find the script with explanations here:
Cheers and input is always welcome fellow coders!

tony piper

I just spent 30 minutes wondering why my browser wasn't getting the gzipped version :-O
If you're using Google Web Access your pages will  be delivered to the browser uncompressed unless you tell GWA to ignore the specific site.
Obvious, really.


I have noticed that if you have the php setting for zlib.output_compression set to 'On' AND you attempt to use hte ob_gzhandler handler to ob_start () the output you get in the browser WILL be garbled in PHP 4.2.3.  I'm guessing that the output buffering is compressing the output to be sent, and then zlib is doing it a second time, but the browser is only uncompressing it once.
But using either one will give you the same results (compressed output from the script)


i had a LOT of trouble trying to get gzipping working correctly
i kept getting a blank page on the first load, although subsequent pages loaded fine.
i tried stuff with ob_flush and ob_end_clean and all kinds of stuff i didn't understand. i'm totally confused about different versions of php and different methods of gzipping.
in the end, i got it working (maybe i'll understand it later).
i ended up using phpBB's code (stripped of non-gz stuff, and ever-so-slightly modified)
echo "<html>\n<head>\n<title>Gzip Test</title>\n<body>\n<h1>testing</h1>\n</body>\n</html>";
gz_header.php - taken form phpBB's page_header.php
$phpver = phpversion();
if ( $phpver >= '4.0.4pl1' && ( strstr($useragent,'compatible') || strstr($useragent,'Gecko') ) )
if ( extension_loaded('zlib') )
else if ( $phpver > '4.0' )
if ( strstr($HTTP_SERVER_VARS['HTTP_ACCEPT_ENCODING'], 'gzip') )
if ( extension_loaded('zlib') )
$do_gzip_compress = TRUE;
header('Content-Encoding: gzip');
gz_footer.php - taken form phpBB's page_tail.php
// Compress buffered output if required and send to browser
if ( $do_gzip_compress )
// Borrowed from!
$gzip_contents = ob_get_contents();
$gzip_size = strlen($gzip_contents);
$gzip_crc = crc32($gzip_contents);
$gzip_contents = gzcompress($gzip_contents, 9);
$gzip_contents = substr($gzip_contents, 0, strlen($gzip_contents) - 4);
echo "\x1f\x8b\x08\x00\x00\x00\x00\x00";
echo $gzip_contents;
echo pack('V', $gzip_crc);
echo pack('V', $gzip_size);
i must admit to not seeing this working in opera 7.11. perhaps i will figure that one out.
if there is anything i should change email me and i can edit my post


I found a site you can use to verify if your gzipping is working:
It tests the gzip'ness of your page.

nicolas dot grekas+php

Here are some precisions :
- the "mode" arg accepts a bit field composed of PHP_OUTPUT_HANDLER_START, PHP_OUTPUT_HANDLER_CONT and PHP_OUTPUT_HANDLER_END. See for an example. The value that jazfresh recommends below (5) is the good one, because 5 == PHP_OUTPUT_HANDLER_START | PHP_OUTPUT_HANDLER_END.
- when called INSIDE an output buffering handler, ob_gzhandler DOES NOT return false when the browser doesn't support compressed pages. It rather returns the original string.

andy dot rem dot ove dot this

From what I can understand, quite some people seem to have problems with this functionality.  If you want to do this quick, like i did, you can use the outstanding class gzip_encode made by Leknor at


Following up on my previous post - it appears that while:
ini_set("zlib.output_compression", "Off"); // with no headers yet sent
// followed by the rest of your code...
did not work in php 4.2.3, it did begin to work as of php 4.3.0 as an override to the php.ini "zlib.output_compression=on" setting. I did not see this in the change log.
In light of this, I'd say using zlib.output_compression is definitely the way to go over ob_gzhandler() in most cases.

sven dot dickert

Following Browsers send
Accept-Encoding: gzip
but have Problems with
Content-Encoding: gzip
* netscape < 5 have problems with CSS/Javascript parts sendet in gzip
* opera 5.x,6 store downloads compressed
* lynx 2.8.4 store downloads compressed but tells this with attaching ".gz" to the filename
You should know about:
* Internet Explorer 5.x,6 should turned to HTTP 1.1 to get "Accept-Encoding: gzip"
* In Mozilla 1.0 RC2 you can write the "Accept-Encoding:" in Preferences/Advanced/HTTP Networking


faisun at sina dot com:
If you read up on output buffering, you'll see that if an output buffer callback returns false, this instructs PHP to output the original string untouched.  That's why ob_gzhandler() returns false if the encoding is not supported.   When ob_start("ob_gzhandler") is used and encoding is not supported, ob_gzhandler() will return false and PHP outputs the original, uncompressed string.
jsnell at e-normous dot com:
Output buffering callback functions accept up to two parameters, so this would probably work better for your situation:
function ob_gz_handler_no_errors($buffer, $mode)
   @ob_gzhandler($buffer, $mode);
However, if you're trying to suppress errors caused by headers already being sent, it would be better to start the output buffering earlier, before any output can be sent.


DON'T compress/gzip your Error pages :
i can think of two good reasons not to compress your error pages:
1. You want to make sure that people can read you error pages, don't you ?
2. Compressing pages have a "side effect" : it's smaller !! if you return "HTTP/1.x 404 Document Not fount" (or any other HTTP error), MSIE will display it's built in error message, not yours if it's small than 513bytes. (see "HOW TO: Turn Off the Internet Explorer 5.x and 6.x 'Show Friendly HTTP Error Messages' Feature on the Server Side";en-us;Q294807 )


Differences between zlib.output_compression and ob_gzhandler:
zlib.output_compression runs in parallel with script execution - it begins compressing as soon as it receives any output from your script, and sends data to the client each time its buffer (4K by default) gets full. ob_gzhandler (actually 'ob_start("ob_gzhandler");') will not start compressing until the script flushes (or, usually, exits), and will in turn send the entire compressed document at once - which makes it more susceptible to causing a perception of latency.
Advantage: zlib.output_compression
On the other hand, ob_gzhandler gives you script-level control allowing you to use it selectively or to unset it after setting it in certain cases. Despite some documentation to the contrary, zlib.output_compression does not appear to be able to be set or unset on a script level; you must instead set it globally (in php.ini) or in your webserver configs or .htaccess files, possibly using FilesMatch-type mechanisms to control which scripts it will or won't apply to - which could get unwieldy for large projects - particularly those which employ PHP to produce images and other non-text output in addition to normal text output.
Advantage: ob_gzhandler
Net advantage: depends on your particular needs. I'm trying zlib.output_compression for now, but I miss the flexibility that ob_gzhandler would provide.
p.s. here's an Apache 1.3.* httpd.conf/.htaccess file snippet that demonstrates the syntax for conditionally setting zlib.output_compression:
<FilesMatch "\.(php|html?)$">
# turn it on with the buffer set to 2K using php_value
php_value zlib.output_compression 2048
# or just turn it on using php_flag
# php_flag zlib.output_compression On


All versions of MSIE have a bug where they don't cache gzipd contents. If your application depends on caching, this is something to keep in mind. In the end, I did:
// These are so benificial, they default to true.
if (!isset($use_page_cache))
   $use_page_cache = 1;
if (!isset($use_compression))
   $use_compression = 1;
// Add browsers here as we must detect them. Opera is an oddball, if we don't detect
// it specifically, it will turn up as MSIE
if (isset($_SERVER['HTTP_USER_AGENT'])) {
   $agent = $_SERVER['HTTP_USER_AGENT'];
   if (eregi("opera",$agent)){
if ($use_compression && !( $use_page_cache && $browser == "msie")) {
   // Turn on compression, makes quite a difference in bandwith usage.
   // However, MSIE (all versions) have a bug with compression and caching. So for MSIE
   // it's either compression or caching. We choose caching.
// ... put stuff in $content ...
if ($use_page_cache) {
   // MD5 is slow, however with a fast server (PIII or better) we should be OK
   $hash = md5($content);
   $headers = getallheaders();
   if (isset($headers['If-None-Match']) && ereg($hash, $headers['If-None-Match']))
header('HTTP/1.1 304 Not Modified');
   header("ETag: \"$hash\"");
print $content;


According to the manual, if ob_gzhander detects that the browser is unable to support deflate or gzip it returns FALSE.  Does this mean that if you use ob_start("ob_gzhandler"), any browsers that do not support gzip/deflate will receive a blank page?
I've been having problems with IE6 displaying gzipped pages and adding a test:
if (substr_count($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip')) ob_start("ob_gzhandler");
else ob_start();
solved the problem.  Surely if ob_gzhandler detects a lack of support for deflation it should return the input unchanged and not FALSE?


About the previous note from Davey:
Does not work. The output size isn’t affected at all, stays the same.
ob_gzhandler compression level use zlib.output_compression_level, which is -1 per default, level 6.
To change the compression level on the fly, simply use ini_set:
ini_set('zlib.output_compression_level', 1);
echo 'This is compressed at level 1';

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