Note that the name ob_implicit_flush is misleading. Despite its name, this function does NOT work with the user output buffer, i.e. the one that the rest of the ob_* functions work with. It will NOT do an automatic ob_flush(). It will do an automatic flush(). Different things.

For example, the following script:

<?php
  ob_implicit_flush();
  for($i = 0; $i < 10; $i++)
  {
    echo "$i\n";
    sleep(1);
  }
?>

will be equivalent to this one:

<?php
  for ($i = 0; $i < 10; $i++)
  {
    echo "$i\n";
    flush();
    sleep(1);
  }
?>

That script will not output anything until the end, if 'output_buffering' is set to 'on' in php.ini. Unfortunately, there is no way to do an implicit ob_flush() after each output, that I am aware of.

If you want the output to come out as it is generated, one solution is to *also* add ob_end_clean() or ob_end_flush() to the beginning of the script:

<?php
  ob_end_flush();
  ob_implicit_flush();
  for ($i = 0; $i < 10; $i++)
  {
    echo "$i\n";
    sleep(1);
  }
?>

This will output as it goes. This is only a problem if you only want one part of the output to come out in real time, and want the rest buffered. In that case, since there's no function to do an implicit ob_flush() every time, you need to call it explicitly. For example, this works:

<?php
  ob_start(); // not needed if output_buffering is on in php.ini
  ob_implicit_flush(); // implicitly calls flush() after every ob_flush()

  echo "This output is buffered.\n";
  echo "As is this.\n";

  for ($i = 0; $i < 10; $i++)
  {
    echo "$i\n";
    ob_flush();
    sleep(1);
  }
?>

Note also that some browsers may wait until they have a certain amount of output. See flush [ http://php.net/manual/en/function.flush.php ] for details. It was my case with Firefox (Iceweasel 17.0); unless I output 1024 bytes at the beginning, it does not begin to output.

It seems like some people are using this to workaround output buffering instead of simply turning it off. To me, if you have an output buffer, it's there for a reason, otherwise you would either do stuff and use ob_flush() or ob_end_flush(), do stuff, and maybe ob_start() again.
Here's how to properly end all output buffers without having to suppress errors (due to there not being any buffers). If you do want a single level of output buffering, I've included a piece for that as well.
<?php
#Get rid of output buffer entirely
while (ob_get_level()) {ob_end_flush();}

#Make sure we have 1 level of output buffering if this isn't CLI.
$notcli=(int)(PHP_SAPI!=='cli');
while (($diff=ob_get_level()-$notcli)!==0) {
    if ($diff>0) {ob_end_flush();}
    else {ob_start();}
}

You can also get the unbuffered output with Linux/Apache without having to do the implicit flush after each line by calling:

ob_implicit_flush(true);
ob_end_flush();

...at the start of the script.

There is another workaround for ob_implicit_flush() in console. Yes, it doesn't works as expected, but you can get similar result by specifying chunk_size=2 in ob_start():

<?php
  ob_start('ob_logstdout', 2);
?>

This will result that every new line (which ends with \n) will flush output buffer.

Hope this will help you.

######### BEWARE ##########

There is a bug (or at least an unexpected feature of ob_implicit_flush) that has been already discussed on the PHP bugtracker :

http://bugs.php.net/bug.php?id=23877
http://bugs.php.net/bug.php?id=16676

Code like this WILL NOT work :

<?
// This will not work as expected on Linux.
ob_implicit_flush (1);
for($i=0;$i<10;$i++) {
   echo "grrrrrrrrrr\n";
   sleep(1);
}
?>

This feature happens on Linux versions of PHP, in all versions of php4 prior to 4.3.3 (don't know yet for the next ones) but also in php5 beta1. ob_implicit_flush has NO EFFECT on command-line (console, CLI) scripts, no flushing at all will be made, all output will be sent at the end of the script.

There is a workaround using ob_end_flush() and ob_flush, here it is :

<?
// This works !
ob_end_flush();
for($i=0;$i<10;$i++) {
  echo "yeah :-))))\n";
  @ob_flush();
  sleep(1);
}
?>

hope this will help. It would have helped me...

From experimenting, it looks like using sessions with session.use_trans_sid=1 will force your output to be buffered regardless of this setting.

My guess is that this is so PHP can hunt for URLs in your output to automatically add the Session ID to them.  It must wait until script output is complete before it starts that replacement, rather than doing it "on the fly".

When i comment out my session_start() line, i get continuous output to the browser.  Put it back in, and i only see the page when it's completely loaded.  Change session.use_trans_sid = 0 and i get continuous output again.