Returning a variable from a function in php (return not working)

return $thisXml;
}
echo $thisXML;

$thisXML; only exists in the scope of the function. Either make $thisXML; global (bad idea) or echo getThisXML() where getThisXML is the function that returns $thisXML;

Are you actually calling the function in the sense of:

$thisXml = getThisXML($someinput);

Maybe a silly question, but I don´t see it in your description.

You need to invoke the function!

$thisXml = 'xml declaration stuff';

echo getThisXML($thisXML);

Or pass the variable by reference:

$thisXml = 'xml declaration stuff';

function getThisXML(&$thisXML){
  ...
  return $thisXml;
}

getThisXML($thisXML);
echo $thisXml;