如何从SimpleXMLElement对象中检索信息

I have an object

class SimpleXMLElement#3 (1) {
     public ${0} =>
     string(10) "idp:088853"
}

retrieved by

$xml = file_get_contents($xml_file); 
$sxml = new SimpleXMLElemet($xml); 
$ids = $sxml->xpath('system-identifier');
foreach( $ids as $id) {
 //....
}

How can I get that string out