get the UUID of an xml with php

Can someone help me to get the UUID of a xml with php?

I am using this function or method

$RE_concepto='<.*?tfd:TimbreFiscalDigital.*?UUID="(.*?)".*?>';
    $xmlCont=file_get_contents("../../files_paso/".$nombreArchivo.".xml");
    preg_match_all('/'.$RE_concepto.'/is',$xmlCont, $matches); 
    $desxml=implode(", ",$matches[1]); 
    unset($matches);

What I do first is save the xml in a step folder and then extract the UUID and if the UUID does not exist then I save the file in the standard folder called files but in this last step it causes me an error when I save to the files folder, does anyone know another way to extract UUID ?