Description:
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It would be extremely handy if, instead of supplying XMLWriter with a uri, we can give it a writeable stream instead, as such;

$h  = fopen('php://output','w');
$xw = new \XMLWriter();
$xw->openStream($h);

This way the XMLWriter does not have to be aware of an output uri.

In my applications (webservices) I simply work with out an output, and an input stream. Different components write to the outputstream, and what they are writing comes from different sources.

Unittests often simply replace the outputstream with php://memory, or php://temp, so the application output can get easily redirected.

Using openUri() I have to break this, which would kind of suck.