$cat = get_the_category();
plouf($cat);
/*
(
    [0] => stdClass Object
        (
            [cat_ID] => 4
            [cat_name] => Clients
            [category_nicename] => clients
            [category_description] => 
            [category_parent] => 0
            [category_count] => 1
            [link_count] => 0
            [posts_private] => 0
            [links_private] => 0
        )

)
*/

echo $cat->cat_ID;
// => rien