<Routes>
  <Route type="source" >
   <Table>
     <Tablename>incoming</Tablename>
     <Fields>
       <Fieldsname ref="dest">truegoto</Fieldsname>
     </Fields>
   </Table>
  </Route>
</Routes>

$doc    = new SimpleXMLElement('routingConfig.xml', null, true);
$foo = $doc->xpath('/Routes/Route[@type="source"]/Table/Tablename[incoming]/following-sibling::Fields/Fieldsname[@ref="dest"]');
print_r($foo);

$foo = $doc->xpath('/Routes/Route[@type="source"]/Table[Tablename="incoming"]/Fields/Fieldsname[@ref="dest"]');