public function keys()
    {
        $keys = [];
        if ($this instanceof \Traversable) {
            foreach ($this as $key => $value) {
                $keys [] = $key;
            }
        }
        return $keys;
    }
public function keys()
    {
        $keys = [];
        if ($this instanceof \Traversable) {
            foreach ($this as $key => $value) {
                $keys [] = $key;
            }
        }
        return $keys;
    }
public function keys ( ) { $ keys = [ ] ; if ( $ this instanceof \ Traversable ) { foreach ( $ this as $ key => $ value ) { $ keys [ ] = $ key ; } } return $ keys ; }
Returns an array with keys from this iterator

@return array
Returns an array with keys from this iterator