XML : XML - Call to a member function children() on a non-object

i have a simple question.

  $url = "http://tel.search.ch/api/....................";  $xml = simplexml_load_file($url);  $namespaces = $xml->getNamespaces(true);  $tel = $xml->entry[0]->children($namespaces["tel"]);  echo $tel->type;  echo $tel->name;    

on line 4 i have this error: "Call to a member function children() on a non-object"

how i can check if tel->type is null or empry?

Thx