如何使用DOMDocument从PHP中提取具有相同名称但不同属性值的元素的节点值？

I have the following XML data :

    <item>     
       <category domain="disease">Ebola</category>
       <category domain="location">Liberia</category>
    </item>

I tried :

 $category=$xmlDoc->getElementsByTagName('item')->getElementsByTagName('category');
 $domain=$category->getAttribute('domain');

However, it gave me a fatal error.