Difference between ob_clean and ob_flush?

The key difference is *_clean() discards changes and *_flush() outputs to the browser.

Usage of ob_end_clean()

it is mostly used when you want to have a chunk of html and do not want to output to the browser right away but may be used in future.

Eg.

ob_start()
echo "<some html chunk>";
$htmlIntermediateData = ob_get_contents();
ob_end_clean();

{{some more business logic}}

ob_start();
echo "<some html chunk>";
$someMoreCode = ob_get_content();
ob_end_clean();

renderTogether($htmlIntermediateCode, $someMoreCode);

where as ob_end_flush() will render twice, once for each.

the *_clean variants just empty the buffer, whereas *_flush functions print what is in the buffer (send the contents to the output buffer).

Example:

ob_start();
print "foo";      // This never prints because ob_end_clean just empties
ob_end_clean();   //    the buffer and never prints or returns anything.

ob_start();
print "bar";      // This IS printed, but just not right here.
ob_end_flush();   // It's printed here, because ob_end_flush "prints" what's in
                  // the buffer, rather than returning it
                  //     (unlike the ob_get_* functions)