Amigo mas claro no puede ser
LEE
$src = new DOMDocument('1.0', 'utf-8');
$src->formatOutput = true;
$src->preserveWhiteSpace = false;
//Loading extern file
$src->load('../xml/Item_component.xml');
//Check each child of first indexed branch node of: <component>
//First get element after root element: <component>
//1st level
$component = $src->
getElementsByTagName('component')->item(0);
//2nd level, get next element after component, here it fails!!
$properties = $component->getElementsByTagName('properties')->item(0);
...
?>