Description

function my_cb($b) {
    ob_start();
    var_dump($b);
    $r = ob_get_clean();
    return $b . $r;
}

ob_start('my_cb');
echo "hello";

Will result in:

Fatal error: ob_start(): Cannot use output buffering in output buffering display handlers

When using:

function my_cb($b) {
    $r = print_r($b, true);
    return $b . $r;
}

it works without error (now, this wasn't the case in earlier PHP versions where it had the same issue as can be seen
in the old docs php/doc-en@e8f4c44#diff-a18323268abbc44bce515215167dfae5d0fb378c63ea36bea0f6e537a6a0114aL115)

What's the reason why print_r with return (which uses buffering internally) can be called in an ob callback now, but ob_start still can't?
Anyway though - given this, var_dump should have a 2nd argument $return to allow returning, so it can be used in ob_start callbacks, where ob_start cannot be used to get it's value