This will show each line at a time with a pause of 2 seconds.
(Tested under IEx and Firefox)

<?php

if (ob_get_level() == 0) ob_start();

for ($i = 0; $i<10; $i++){

        echo "<br> Line to show.";
        echo str_pad('',4096)."\n";   

        ob_flush();
        flush();
        sleep(2);
}

echo "Done.";

ob_end_flush();

?>

For a Windows system using IIS, the ResponseBufferLimit takes precedence over PHP's output_buffering settings. So you must also set the ResponseBufferLimit to be something lower than its default value.

For IIS versions older than 7, the setting can be found in the %windir%\System32\inetsrv\fcgiext.ini file (the FastCGI config file). You can set the appropriate line to:
  ResponseBufferLimit=0

For IIS 7+, the settings are stored in %windir%\System32\inetsrv\config. Edit the applicationHost.config file and search for PHP_via_FastCGI (assuming that you have installed PHP as a FastCGI module, as per the installation instructions, with the name PHP_via_FastCGI). Within the add tag, place the following setting at the end:
  responseBufferLimit="0"
So the entire line will look something like:
  <add name="PHP_via_FastCGI" path="*.php" verb="*" modules="FastCgiModule" scriptProcessor="C:\PHP\php-cgi.exe" resourceType="Either" responseBufferLimit="0" />
Alternatively you can insert the setting using the following command:
  %windir%\system32\inetsrv\appcmd.exe set config /section:handlers "/[name='PHP_via_FastCGI'].ResponseBufferLimit:0"

This is what I use to turn off pretty much anything that could cause unwanted output buffering and turn on implicit flush:

<?php

    @apache_setenv('no-gzip', 1);
    @ini_set('zlib.output_compression', 0);
    @ini_set('implicit_flush', 1);
    for ($i = 0; $i < ob_get_level(); $i++) { ob_end_flush(); }
    ob_implicit_flush(1);

?>

If it still fails though, keep in mind that Internet Explorer and Safari have a 1k buffer before incremental rendering kicks in, so you'll want to output some padding as well.

On Windows xampp 1.3 with php 4.3.4 is use this functions
to force a flush.
<?php
function dummyErrorHandler ($errno, $errstr, $errfile, $errline) {
}

function forceFlush() {   
    ob_start();
    ob_end_clean();
    flush();
    set_error_handler("dummyErrorHandler");
    ob_end_flush();
    restore_error_handler();
}
?>

ob_end_flush generates a warning, which is supressed, using the dummy-errorhander. You could also use @, but then nusphere will also print the warning.

Like IE, Safari needs a fair amount of data before it'll display anything, actually more than explorer.  The following code works for me in Firefox and Safari, and should work in IE as well.

<?php

for($i = 0; $i < 40000; $i++)
{
echo ' '; // extra spaces
}
// keeps it flowing to the browser…
flush();
// 50000 microseconds keeps things flowing in safari, IE, firefox, etc
usleep(50000);

?>

This code came from a comment on a blog discussing browser functionality with flush();

IE 7, Opera 9.6, any. All is quite simple. Today I just fogot what i do every night, :)

<?php
// All you need is 256 spaces first
echo str_repeat(" ", 256)."<pre>"; flush();

// and ANY TAG before \r\n
echo "working...<br/>\r\n"; flush(); sleep(1); // this in cycle
?>

It is a bit complicated to work with the funktion flush() and you have to experiment with it a bit.
So if you design a site which has a timeloop at the end that calls a other site via a form data input (Data Submit) you have
to give something out to the buffer to get that new site loaden quick.

For example:

<?php
$instant=gettimeofday();
$timenow=$instant["sec"];//Start Time

//timeloop(e.g. for security_save after 30 min)
while (1) { echo "<b></b>";//Useless (only to quickload next
                                    //or same Site when do a switch)
flush();                          //giveout buffer
$instant=gettimeofday();
$timeactual=$instant["sec"]; //get Actual Time in Secs
$flag=(($timeactual>$timenow+$diff)? 1:0);//$diff=switchTime
if ($flag) { what_do_at_switch_Time();//Sec.Save etc.etc.
$timenow=$timeactual; } //Set new Start Time
sleep(5); //Or so...(Important)
} //End of while-Loop
?>

So you can programm a security save or other function in your site and if you do a switch the upload of the new or same site (the called site) works...

In my testing, Internet Explorer 6.0 wouldn't flush anything nested in <table> or <td> tags, regardless of padding. But at the <body> level everything flushed with no fuss -- no padding or tags required.

Both Firefox 1.0 and Safari 2.0 could flush within tables, and both required a tag after the text (like <br>). Safari could flush only after the first 1024 characters where received. Firefox needed at least 8 characters per flush (but it could flush anything at the <body> level).

So the only thing that worked on all those browsers was this:

<html>
<body>
<?php  // not in table tags for IE
echo str_pad('',1024);  // minimum start for Safari
for ($i=10; $i>0; $i--) {
    echo str_pad("$i<br>\n",8);
    // tag after text for Safari & Firefox
    // 8 char minimum for Firefox
    flush();  // worked without ob_flush() for me
    sleep(1);
}
?>
</body>
</html>