create function to call category name and slug

You don’t need to hook the dw_get_category() function. Simply do it like this:

  function dw_get_category() {
    global $post;
    $categories = get_the_category($post->ID);
        if ( $categories ) {
           foreach ($categories as $category) {
               $dw_category_slug = $category->slug;
               $dw_category_name = $category->name;
           }
        } else {
            $dw_category_slug = 'utopia';
        }         
    $dw_category = array('name' => $dw_category_name, 'slug' =>    $dw_category_slug);
    return $dw_category;     
    } 

Then call it like this:

$the_post_cats = dw_get_category();
echo 'Category: '.$the_post_cats['name'];
echo 'Slug: '.$the_post_cats['slug'];