ob_start

(PHP 4, PHP 5, PHP 7, PHP 8)

ob_start — Turn on output buffering

Description

ob_start(?callable $callback = null, int $chunk_size = 0, int $flags = PHP_OUTPUT_HANDLER_STDFLAGS): bool

This function will turn output buffering on. While output buffering is active no output is sent from the script, instead the output is stored in an internal buffer. See What Output Is Buffered? on exactly what output is affected.

Output buffers are stackable, that is, ob_start() may be called while another buffer is active. If multiple output buffers are active, output is being filtered sequentially through each of them in nesting order. See Nesting Output Buffers for more details.

See User-Level Output Buffers for a detailed description of output buffers.

Parameters

callback

An optional callback callable may be specified. It can also be bypassed by passing null.

callback is invoked when the output buffer is flushed (sent), cleaned, or when the output buffer is flushed at the end of the script.

The signature of the callback is as follows:

handler(string $buffer, int $phase = ?): string

buffer: Contents of the output buffer.
phase: Bitmask of PHP_OUTPUT_HANDLER_* constants . See Flags Passed To Output Handlers for more details.

If callback returns false the contents of the buffer are returned. See Output Handler Return Values for more details.

Warning

Calling any of the following functions from within an output handler will result in a fatal error: ob_clean(), ob_end_clean(), ob_end_flush(), ob_flush(), ob_get_clean(), ob_get_flush(), ob_start().

See Output Handlers and Working With Output Handlers for more details on callbacks (output handlers).

chunk_size

If the optional parameter chunk_size is passed, the buffer will be flushed after any block of code resulting in output that causes the buffer's length to equal or exceed chunk_size. The default value 0 means that all output is buffered until the buffer is turned off. See Buffer Size for more details.

flags

The flags parameter is a bitmask that controls the operations that can be performed on the output buffer. The default is to allow output buffers to be cleaned, flushed and removed, which can be set explicitly via the buffer control flags . See Operations Allowed On Buffers for more details.

Each flag controls access to a set of functions, as described below:

Constant	Functions
`PHP_OUTPUT_HANDLER_CLEANABLE`	ob_clean()
`PHP_OUTPUT_HANDLER_FLUSHABLE`	ob_flush()
`PHP_OUTPUT_HANDLER_REMOVABLE`	ob_end_clean(), ob_end_flush(), ob_get_clean(), ob_get_flush()

Note: Prior to PHP 8.4.0, the flags parameter could set the output handler status flags as well.

Return Values

Returns true on success or false on failure.

Examples

Example #1 User defined callback function example

<?php

function callback($buffer)
{
  // replace all the apples with oranges
  return (str_replace("apples", "oranges", $buffer));
}

ob_start("callback");

?>
<html>
<body>
<p>It's like comparing apples to oranges.</p>
</body>
</html>
<?php

ob_end_flush();

?>

The above example will output:

<html>
<body>
<p>It's like comparing oranges to oranges.</p>
</body>
</html>

Example #2 Creating an unerasable output buffer

<?php

ob_start(null, 0, PHP_OUTPUT_HANDLER_STDFLAGS ^ PHP_OUTPUT_HANDLER_REMOVABLE);

?>

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User Contributed Notes 6 notes

down

Ray Paseur (Paseur ... ImagineDB.com) ¶

19 years ago

You can use PHP to generate a static HTML page.  Useful if you have a complex script that, for performance reasons, you do not want site visitors to run repeatedly on demand.  A "cron" job can execute the PHP script to create the HTML page.  For example:

<?php // CREATE index.html
   ob_start();
/* PERFORM COMLEX QUERY, ECHO RESULTS, ETC. */
   $page = ob_get_contents();
   ob_end_clean();
   $cwd = getcwd();
   $file = "$cwd" .'/'. "index.html";
   @chmod($file,0755);
   $fw = fopen($file, "w");
   fputs($fw,$page, strlen($page));
   fclose($fw);
   die();
?>

down

net_navard at yahoo dot com ¶

18 years ago

Hello firends

ob_start() opens a buffer in which all output is stored. So every time you do an echo, the output of that is added to the buffer. When the script finishes running, or you call ob_flush(), that stored output is sent to the browser (and gzipped first if you use ob_gzhandler, which means it downloads faster). 

The most common reason to use ob_start is as a way to collect data that would otherwise be sent to the browser.

These are two usages of ob_start():

1-Well, you have more control over the output. Trivial example: say you want to show the user an error message, but the script has already sent some HTML to the browser. It'll look ugly, with a half-rendered page and then an error message. Using the output buffering functions, you can simply delete the buffer and sebuffer and send only the error message, which means it looks all nice and neat buffer and send 
2-The reason output buffering was invented was to create a seamless transfer, from: php engine -> apache -> operating system -> web user

If you make sure each of those use the same buffer size, the system will use less writes, use less system resources and be able to handle more traffic. 

With Regards, Hossein

down

ed.oohay (a) suamhcs_rodnan ¶

21 years ago

Output Buffering even works in nested scopes or might be applied in recursive structures... thought this might save someone a little time guessing and testing :)

<pre><?php
    
    ob_start();              // start output buffer 1
    echo "a";                // fill ob1
        
        ob_start();              // start output buffer 2
        echo "b";                // fill ob2
        $s1 = ob_get_contents(); // read ob2 ("b")
        ob_end_flush();          // flush ob2 to ob1
        
    echo "c";                // continue filling ob1
    $s2 = ob_get_contents(); // read ob1 ("a" . "b" . "c")
    ob_end_flush();          // flush ob1 to browser
    
    // echoes "b" followed by "abc", as supposed to:
    echo "<HR>$s1<HR>$s2<HR>";
    
?></pre>

... at least works on Apache 1.3.28

Nandor =)

down

Asher Haig (ahaig at ridiculouspower dot com) ¶

17 years ago

When a script ends, all buffered output is flushed (this is not a bug: http://bugs.php.net/bug.php?id=42334&thanks=4). What happens when the script throws an error (and thus ends) in the middle of an output buffer? The script spits out everything in the buffer before printing the error!

Here is the simplest solution I have been able to find. Put it at the beginning of the error handling function to clear all buffered data and print only the error:

$handlers = ob_list_handlers();
while ( ! empty($handlers) )    {
    ob_end_clean();
    $handlers = ob_list_handlers();
}

down

Chris ¶

14 years ago

Careful with while using functions that change headers of a page; that change will not be undone when ending output buffering.

If you for instance have a class that generates an image and sets the appropriate headers, they will still be in place after the end of ob.

For instance:
<?php
  ob_start();
  myClass::renderPng(); //header("Content-Type: image/png"); in here
  $pngString = ob_get_contents();
  ob_end_clean();
?>

will put the image bytes into $pngString, and set the content type to image/png. Though the image will not be sent to the client, the png header is still in place; if you do html output here, the browser will most likely display "image error, cannot be viewed", at least firefox does.

You need to set the correct image type (text/html) manually in this case.

down

ernest at vogelsinger dot at ¶

19 years ago

When you rely on URL rewriting to pass the PHP session ID you should be careful with ob_get_contents(), as this might disable URL rewriting completely.

Example:
ob_start();
session_start();
echo '<a href=".">self link</a>';
$data = ob_get_contents();
ob_end_clean();
echo $data;

In the example above, URL rewriting will never occur. In fact, rewriting would occur if you ended the buffering envelope using ob_end_flush(). It seems to me that rewriting occurs in the very same buffering envelope where the session gets started, not at the final output stage.

If you need a scenario like the one above, using an "inner envelope" will help:

ob_start();
ob_start();   // add the inner buffering envelope
session_start();
echo '<a href=".">self link</a>';
ob_end_flush(); // closing the inner envelope will activate URL rewriting
$data = ob_get_contents();
ob_end_clean();
echo $data;

In case you're interested or believe like me that this is rather a design flaw instead of a feature, please visit bug #35933 (http://bugs.php.net/bug.php?id=35933) and comment on it.

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ob_start

Description

Parameters

Return Values

Examples

See Also

Found A Problem?

User Contributed Notes 6 notes