ob_start

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();
?>

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

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 =)

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();
}

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.

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.