Because there is no way of removing the callback from the OB once you've set it, the only way to stop the callback function from having any effect is to do something like:

<?php
$ignore_callback = false;
ob_start('my_callback');
...
if($need_to_abort) {
   $ignore_callback = true;
   ob_end_clean();
   ...
}

Example:

<?php

ob_start();

$output = ob_get_contents();

ini_set('zlib.output_compression', '1');

ob_end_clean();

If you would like to clear the output buffer, but not the headers (because you use firephp for example...), than this is the solution:

<?php
   ...

    $headers = array();
    if ( !headers_sent() ) {
      $headers = apache_response_headers();
    }

    ob_end_clean();
    ob_start();

    if ( !empty( $headers ) ) {
      foreach ( $headers as $name => $value ) {
        header( "$name: $value" );
      }
    }

   ...
?>

I use it in a general exception handler in a web application, where I clear the buffer (but not the debug-info-containing headers), and send a 500 error page with readfile().

Good PHPing,

echo "<p>Search running. Please be patient. . .";
$output = "<p>FileList: </p>n";
if (is_dir($dir)) {
  $dh = opendir($dir);

  while (($fd = readdir($dh)) != false) {
    echo " .";
    $output .= $fd;
  }
}
echo "</br>Search Complete!</p>n";
echo $output;

In addition to John Smith's comment (#42939), ob_gzhandler() may still set the HTTP header "Content-Encoding" to "gzip" or "deflate" even if you call ob_end_clean(). This will cause a problem in the following situation:

1. Call ob_gzhandler().
2. Echo "Some content";
3. Call ob_end_clean().
4. Echo "New content";

In the above case, the browser may receive the "Content-Encoding: gzip" HTTP header and attempts to decompress the uncompressed "New content". The browser will fail.

In the following situation, this behaviour will go unnoticed:

1. Call ob_gzhandler().
2. Echo "Some content";
3. Call ob_end_clean().
4. Call ob_gzhandler().
5. Echo "New content";

This is because the second ob_gzhandler() will mask the absence of the first ob_gzhandler().

ob_start();

$mtime=filemtime($_SERVER["SCRIPT_FILENAME"])-date("Z");
$gmt_mtime = date('D, d M Y H:i:s', $mtime) . ' GMT';

$headers = getallheaders();

if(isset($headers["If-Modified-Since"])) {
    if ($headers["If-Modified-Since"] == $gmt_mtime) {
        header("HTTP/1.1 304 Not Modified");
        ob_end_clean();
        exit;
    }
}

$size=ob_get_length();
header("Last-Modified: ".$gmt_mtime);
header("Content-Length: $size");
ob_end_flush();

Instead of checking the If-Modified-Since-Header against the date of the last modification of the script, you can of course query a database or take any other date that is somehow related to the modification of the result of your script.

echo "<p>Search running. Please be patient. . .";
$output = "<p>FileList: </p>n";
if (is_dir($dir)) {
   $dh = opendir($dir);

   while (($fd = readdir($dh)) != false) {
      echo " .";
      ob_start();
      echo $fd;
      $output .= ob_get_contents();
      ob_end_clean();
    }
}
echo "</br>Search Complete!</p>n";
echo $output;

<?php

function ClearBuffer($Buffer) {
    return "";
}

function Shutdown() {
    ob_start("ClearBuffer");
}

register_shutdown_function("Shutdown");

<?php