Mine

$cat = get_the_category();plouf($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