The definition should mention that the function also "turns off output buffering", not just cleans it.

Also, don't forget that you will need to ob_start() again for any successive calls:

<?php
ob_start();
echo "1";
$content = ob_get_clean();

ob_start(); // This is NECESSARY for the next ob_get_clean() to work as intended.
echo "2";
$content .= ob_get_clean();

echo $content;
?>

Output: 12

Without the second ob_start(), the output is 21 ...

Keep in mind that output may be buffered by default, depending on how you are running PHP (CGI, CLI, etc.).   You can use ob_get_level() to determine if an output buffer has already been started.  On most web servers I've used, output buffering is already one level deep before my scripts start running.

You should only end as many output buffers as you start.  Assuming that your buffer is always the first buffer, or otherwise closing pre-existing buffers, could lead to problems.  In PHP 5.5, you can ensure that output buffers are ended properly using a try-finally block.

Something like this is almost guaranteed to break stuff:

<?php
// Don't ever do this!
while (ob_get_level() > 1)
{
    ob_end_flush();
}

$content = ob_get_clean();
?>

The problem is that number, "1".  Using a fixed number there is asking for trouble.  Instead, use ob_get_level() to get the number of output buffers applied when your code starts, and return to that number, if you really must use an unknown number of output buffers:

<?php
ob_start();
$saved_ob_level = ob_get_level();

// Do stuff here:
run_something();

// If you really must close all of your output buffers except one, this'll do it:
while (ob_get_level() > $start_ob_level)
{
    ob_end_flush();
}

// And now, the final output buffer that belongs to us:
$content = ob_get_clean();
?>