如何从多维数组中获取值？

I want to get the value of array element ad_id from a multidimensional array. When I print this array using print_r($xmls) I get this array:

.....
 [5] => SimpleXMLElement Object
                (
                    [@attributes] => Array
                        (
                            [ad_id] => 139293
                        )

                    [price] => 400,000
                    [price_comment] => SimpleXMLElement Object
                        (
                        )

                    [location] => Northern NSW
                    [new_used] => Used
                    [descript] => Ex fishing fleet mother ship, fully equipped for long range work. 
Ideal live aboard cruising /
                    [Resources] => SimpleXMLElement Object
                        (
                            [image] => //imgs.yachthub.com/1/3/9/2/9/3/0_2.jpg
                        )

                )

            [6] => SimpleXMLElement Object
                (
                    [@attributes] => Array
                        (
                            [ad_id] => 130247
                        )

                    [price] => POA
                    [price_comment] => SimpleXMLElement Object
                        (
                        )

                    [location] => Athens, Greece
                    [new_used] => Used
                    [descript] => Luxury motor yacht O’RAMA is 53,54m (175' 7") in length and was built by Golden Yachts in 2009
                    [Resources] => SimpleXMLElement Object
                        (
                            [image] => //imgs.yachthub.com/1/3/0/2/4/7/0_2.jpg
                        )

                )
.....

(It is a much larger array, I am writing only two elements here.) I want to get ad_id. I can get value of image using $xmls->Resources->image; but in case of ad_id, it is not working. Maybe because of special character in @attributes?

$var = "@attributes";
$xmls->$var['ad_id'];

This should work