Description:
------------
We currently use an exception-handler that writes information about the exception to a xml file.
Recently we noticed, that sometimes the resulting xml file is just empty - without any error message being triggered.

I was able to reduce the code to the example below. There's no obvious reason why the script should not be able to write the xml content.

Interesting thing is: Throwing an exception manually does not trigger this behavior.

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

@unlink( '/tmp/err.xml' );

function exception_handler( Throwable $exception )
{
    echo "exception handler called\n";
    $xw = new XMLWriter();

    var_dump( $xw );

    $xw->openURI( '/tmp/err.xml' );

    $xw->startDocument( '1.0', 'UTF-8' );

    $xw->startElement( 'error' );
    $xw->endElement();

    $returnValue = $xw->endDocument();
    if ( $returnValue === false )
    {
        die( "unable to write error xml\n" );
    }

    die("error xml written\n");

} # function exception_handler(...)

set_exception_handler( 'exception_handler' );

new SoapClient( 'https://localhost/', [] );
#throw new Exception("dummy");

Expected result:
----------------
exception handler called
object(XMLWriter)#1 (0) {
}
error xml written

(/tmp/err.xml exists with content <?xml version="1.0" encoding="UTF-8"?>
<error/>)


Actual result:
--------------
exception handler called
object(XMLWriter)#1 (0) {
}
unable to write error xml

(/tmp/err.xml exists but is empty)