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PHP : Function Reference : Output Control Functions

Output Control Functions


The Output Control functions allow you to control when output is sent from the script. This can be useful in several different situations, especially if you need to send headers to the browser after your script has began outputting data. The Output Control functions do not affect headers sent using header() or setcookie(), only functions such as echo() and data between blocks of PHP code.


When upgrading from PHP 4.1.x (and 4.2.x) to 4.3.x due to a bug in earlier versions you must ensure that implict_flush is OFF in your php.ini, otherwise any output with ob_start() will not be hidden from output.


No external libraries are needed to build this extension.


There is no installation needed to use these functions; they are part of the PHP core.

Runtime Configuration

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

Table 236. Output Control configuration options

Name Default Changeable Changelog
output_buffering "0" PHP_INI_PERDIR  
output_handler NULL PHP_INI_PERDIR Available since PHP 4.0.4.
implicit_flush "0" PHP_INI_ALL PHP_INI_PERDIR in PHP <= 4.2.3.

For further details and definitions of the PHP_INI_* constants, see the Appendix I, php.ini directives.

Here's a short explanation of the configuration directives.

output_buffering boolean/integer

You can enable output buffering for all files by setting this directive to 'On'. If you wish to limit the size of the buffer to a certain size - you can use a maximum number of bytes instead of 'On', as a value for this directive (e.g., output_buffering=4096). As of PHP 4.3.5, this directive is always Off in PHP-CLI.

output_handler string

You can redirect all of the output of your scripts to a function. For example, if you set output_handler to mb_output_handler(), character encoding will be transparently converted to the specified encoding. Setting any output handler automatically turns on output buffering.


You cannot use both mb_output_handler() with ob_iconv_handler() and you cannot use both ob_gzhandler() and zlib.output_compression.


Only built-in functions can be used with this directive. For user defined functions, use ob_start().

implicit_flush boolean

FALSE by default. Changing this to TRUE tells PHP to tell the output layer to flush itself automatically after every output block. This is equivalent to calling the PHP function flush() after each and every call to print() or echo() and each and every HTML block.

When using PHP within an web environment, turning this option on has serious performance implications and is generally recommended for debugging purposes only. This value defaults to TRUE when operating under the CLI SAPI.

See also ob_implicit_flush().

Resource Types

This extension has no resource types defined.

Predefined Constants

This extension has no constants defined.


Example 1682. Output Control example



setcookie("cookiename", "cookiedata");



In the above example, the output from echo() would be stored in the output buffer until ob_end_flush() was called. In the mean time, the call to setcookie() successfully stored a cookie without causing an error. (You can not normally send headers to the browser after data has already been sent.)

See Also

See also header() and setcookie().

Table of Contents

flush — Flush the output buffer
ob_clean — Clean (erase) the output buffer
ob_end_clean — Clean (erase) the output buffer and turn off output buffering
ob_end_flush — Flush (send) the output buffer and turn off output buffering
ob_flush — Flush (send) the output buffer
ob_get_clean — Get current buffer contents and delete current output buffer
ob_get_contents — Return the contents of the output buffer
ob_get_flush — Flush the output buffer, return it as a string and turn off output buffering
ob_get_length — Return the length of the output buffer
ob_get_level — Return the nesting level of the output buffering mechanism
ob_get_status — Get status of output buffers
ob_gzhandler — ob_start callback function to gzip output buffer
ob_implicit_flush — Turn implicit flush on/off
ob_list_handlers — List all output handlers in use
ob_start — Turn on output buffering
output_add_rewrite_var — Add URL rewriter values
output_reset_rewrite_vars — Reset URL rewriter values

Code Examples / Notes » ref.outcontrol


[Concerns IE refusing to jump to a #something in the URL.]
I encoutered a bug in IE6/W2000 that can be solved by turning output buffering on.
Maybe it also helps in other situations/M$-OS, not sure.
A page with a hash in the URL, and IE doesn't jump to that location.
- In test.php the anchortag is placed normally like:
<a name="something">
- test.php takes a few seconds to load because of heavy-duty database activity.
IE just ignores the hash #something.
It looks like IE 'forgets' the hash if it hasn't encoutered it YET in the HTML.
Turning output buffering on resolves that issue.

trucex um,

Unfortunately, the PHP guys didn't build support into any of the image output functions to return the image instead of outputting it.
Fortunately, we have output buffering to fix that.
$im = imagecreatetruecolor(200, 200);
// Other image functions here...
$imageData = ob_get_contents();
You can now use the $imageData variable to either create another GD image, save it, put it in a database, make modifications to the binary, or output it to the user. You can easily check the size of it as well without having to access the disk...just use strlen();


Trying to benchmark your server when using output_buffering ?
Don't forget that the value 4096 in the php.ini will give you complete different loadtimes compares to the value of 1.
In the first case the output will be sent after buffering 4096 and the loadtime timed at the end of the page will contain the loadtime needed to download the complete page in the clientbrowser while the second value will contain the loadtime needed to place the complete page in the buffer. The time needed for sending is not clocked.
This can be very frustrating if you don't see the differance between server and the 1st is using 4096 instead of 1.
Although technically much faster than the second server the second server was providing much better loadtime results.
This result will grow when using large amounts of output.
But this becomes interesting if you want to measure the time needed for the page to be loaded for the client.

nobbie @t php d0t net

There is a problem in MSIE 5.5,6 with regards to Page compression. Users might experience pages not loading completely, or just a blank page.
This articles you are looking for is what you're looking for:
Microsoft Knowledge Base Article - 312496 (for MSIE 6)
Microsoft Knowledge Base Article - 313712 (for MSIE 5.5)
It states that you should upgrade to the latest MSIE Service Pack to fix the following problem:
Internet Explorer May Lose the First 2,048 Bytes of Data That Are Sent Back from a Web Server That Uses HTTP Compression


Sometimes you might not want to include a php-file under the specifications defined in the functions include() or require(), but you might want to have in return the string that the script in the file "echoes".
Include() and require() both directly put out the evaluated code.
For avoiding this, try output-buffering:
$result = ob_get_contents();
$result = ob_get_contents();
which i consider the same, correct me if I'm wrong.
Best regards, BasicArtsStudios


Output buffering is set to '4096' instead of 'Off' or '0' by default in the php-5.0.4-10.5 RPM for Fedora Core release 4 (Stentz).  This has cost me much time!


Now this just blew my mind. I had a problem with MySQL being incredibly slow on Windows 2003 running IIS... on ASP/VBScript pages. PHP is also installed on the server and so is Microsoft SQL 2005 Express. (Yes, we're running ASP, PHP, MySQL and MS SQL on the same Windows 2003 Server using IIS.)
I was browsing the internet for a solution and saw a suggestion that I change output_buffering to on if MySQL was slow for PHP pages.  Since we also served PHP pages with MySQL from the same server, it caught my eye.  For the hell of it, I went into php.ini and changed output_buffering to on and suddenly MySQL and ASP was faster... MySQL and PHP was faster... Microsoft SQL Server 2005 Express and ASP was faster.... everything was faster... even stuff that had no PHP!
And I didn't even have to restart IIS. As soon as I saved the php.ini file with the change, everything got faster.
Apparently PHP and MySQL and IIS are so intertwined somehow that changing the buffering setting really effects the performance of the entire server.
So, if you are having performance problems on Windows 2003 & IIS, you might try setting output_buffering = On in php.ini if you happen to have PHP installed.  Having it set to off apparently effects the performance of Windows 2003 and IIS severely... even for webpages that do not use PHP or MySQL.

jgeewax a t gmail

It seems that while using output buffering, an included file which calls die() before the output buffer is closed is flushed rather than cleaned. That is, ob_end_flush() is called by default.
// a.php (this file should never display anything)
// b.php
print "b";
This ends up printing "b" rather than nothing as ob_end_flush() is called instead of ob_end_clean(). That is, die() flushes the buffer rather than cleans it. This took me a while to determine what was causing the flush, so I thought I'd share.


In re to erwinX at darwineX dot nl:
Adding an ampersand (&) before the hash seems to work too (for me at least), i.e.: http://somedomain.tld/blah.php?arg=x&#something
I guess php then interperts it as an argument to the script. Might save some time and resources.


I ran out of memory, while output buffering and drawing text on imported images. Only the top portion of the 5MP image was displayed by the browser.  Try increasing the memory limit in either the php.ini file( memory_limit = 16M; ) or in the .htaccess file( php_value memory_limit "16M" ). Also see function memory_get_usage() .


For those who are looking for optimization, try using buffered output.
I noticed that an output function call (i.e echo()) is somehow time expensive. When using buffered output, only one output function call is made and it seems to be much faster.
Try this :
for ($i = 0; $i < 5000; $i++)
echo str_repeat ("your string blablabla bla bla", (rand() % 4) + 1)."
echo your_benchmark_end_function();
And then :
ob_start ();
for ($i = 0; $i < 5000; $i++)
echo str_repeat ("your string blablabla bla bla", (rand() % 4) + 1)."
echo your_benchmark_end_function();
ob_end_flush ();


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