Description:
------------
Output buffering inside output buffering handler is currently forbidden.

Ideally, this limitation could be removed, but as this may be too much work, I mostly really miss some way to test whether my code is running inside an output buffering handler or not (for example in a custom error handler).

PHP really miss a way to check for this situation.

Currently, when using output buffering in an output buffering handler context, an E_ERROR is thrown. Could it be possible to trigger an E_RECOVERABLE_ERROR instead? This would allow me to catch the situation and degrade gracefully.

Or maybe a new special function "ob_in_handler()", returning a boolean, is a better idea ?

Test script:
---------------
<?php

ob_start('abc');

function abc()
{
    ob_start();
}

?>

Expected result:
----------------
PHP Catchable fatal error:  ob_start(): Cannot use output buffering in output buffering display handlers in [...] on line 5

Actual result:
--------------
PHP Fatal error:  ob_start(): Cannot use output buffering in output buffering display handlers in [...] on line 5