Use the (string) method to cast a type in to the type string. For example:
$x = (string) $stdVar->relationship->source->screen_name;
var_dump($x); // will output string(n) "damisleq"
by the way, if you get the result from Twtter as JSON you can also use associative array function in json_decode. For example. $result = json_decode($string, true); the true states you want a array as result in stead of a stdObject.