Description:
------------
Shutdown functions are not called when exit is called from output handler

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

// capture any unexpected output and exit execution
ob_start(function($buffer) {
    echo 'unexpected output: ' . $buffer . "\n";
    exit;
}, 1);

// do some work on shutdown
register_shutdown_function(function() {
    ob_flush();
    ob_end_clean();
    echo 'shutting down';
});

echo 'x';

Expected result:
----------------
unexpected output: x
shutting down

Actual result:
--------------
(empty result)