SimpleXMLElement and Warning: Illegal offset type

Warning: Illegal offset type

i have this var from an xml output

var_dump($key);

its give me

object(SimpleXMLElement)#11 (1) { [0]=> string开发者_运维技巧(5) "Cairo" } 

now i want make "Cairo" as a key to get its value like

$lang[] = array('Cairo' => "Cairo city");

the error appear when do this

echo $lang[$key];

and its give me the "Warning: Illegal offset type" because of the key is SimpleXMLElement how i can make it normal var?

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Cast it to a string with (string)$key.