Note for mod_fcgid users (please, use at your own risk).

Quick Solution

The accepted answer of Joeri Sebrechts is indeed functional. However, if you use mod_fcgid you may find that this solution does not work on its own. In other words, when the flush function is called the connection to the client does not get closed.

The FcgidOutputBufferSize configuration parameter of mod_fcgid may be to blame. I have found this tip in:

1. this reply of Travers Carter and
2. this blog post of Seumas Mackinnon.

After reading the above, you may come to the conclusion that a quick solution would be to add the line (see “Example Virtual Host” at the end):

FcgidOutputBufferSize 0

in either your Apache configuration file (e.g, httpd.conf), your FCGI configuration file (e.g, fcgid.conf) or in your virtual hosts file (e.g., httpd-vhosts.conf).

In (1) above, a variable named “OutputBufferSize” is mentioned. This is the old name of the FcgidOutputBufferSize mentioned in (2) (see the upgrade notes in the Apache web page for mod_fcgid).

Details & A Second Solution

The above solution disables the buffering performed by mod_fcgid either for the whole server or for a specific virtual host. This might lead to a performance penalty for your web site. On the other hand, this may well not be the case since PHP performs buffering on its own.

In case you do not wish to disable mod_fcgid‘s buffering there is another solution… you can force this buffer to flush.

The code below does just that by building on the solution proposed by Joeri Sebrechts:

<?php
    ob_end_clean();
    header("Connection: close");
    ignore_user_abort(true); // just to be safe
    ob_start();
    echo('Text the user will see');

    echo(str_repeat(' ', 65537)); // [+] Line added: Fill up mod_fcgi's buffer.

    $size = ob_get_length();
    header("Content-Length: $size");
    ob_end_flush(); // Strange behaviour, will not work
    flush(); // Unless both are called !
    // Do processing here 
    sleep(30);
    echo('Text user will never see');
?>

What the added line of code essentially does is fill up mod_fcgi‘s buffer, thus forcing it to flush. The number “65537” was chosen because the default value of the FcgidOutputBufferSize variable is “65536”, as mentioned in the Apache web page for the corresponding directive. Hence, you may need to adjust this value accordingly if another value is set in your environment.

My Environment

• WampServer 2.5
• Apache 2.4.9
• PHP 5.5.19 VC11, x86, Non Thread Safe
• mod_fcgid/2.3.9
• Windows 7 Professional x64

Example Virtual Host

<VirtualHost *:80>
    DocumentRoot "d:/wamp/www/example"
    ServerName example.local

    FcgidOutputBufferSize 0

    <Directory "d:/wamp/www/example">
        Require all granted
    </Directory>
</VirtualHost>