Description:
------------
If ob_start() callback returns an empty string then nothing should be printed. This is the current behavior in PHP 5.3. Behavior in PHP 5.4 is different - if a callback returns an empty string then the result is same as if it wouldn't be called at all - next call is prepended by last call value.

So there is no way for callback how to discard the output.


Test script:
---------------
ob_start(function ($s) {
	fwrite(STDERR, strlen($s) . "\n");
	if ($s == "b\n") {
		return $s;
	}
	return "";
}, 2);
echo "a\n";
echo "b\n";
exit;


Expected result:
----------------
2
2
b
0


Actual result:
--------------
2
4
4