Further to rmirabelle's comment, to compare two SimpleXML elements, do the comparison between the objects returned by the dom_import_simplexml function, as per:
<?php
if(dom_import_simplexml($simplexml_element_1)===dom_import_simplexml($simplexml_element_2)){
...
}
?>
You must ensure that both are SimpleXML elements before conversion to DOM objects.