getting the right cats

$allposts = get_posts('post_type=post&numberposts=100'); //get a lot of recent posts
$cats = array(); $cat = array(); //initialize arrays
foreach($allposts as $allpost) {
$postcats = get_the_category($allpost->ID); //get all post's categories
foreach($postcats as $ca) { $cat[] = $ca->term_id; } $postcats = $cat; $cat = array(); //this uses less queries than wp_get_post_categories()
$cats = array_merge($cats,$postcats);
$cats = array_unique($cats); } //only keep unique category IDs

$allcats = get_categories(); //get all categories of the site, in case tha latest posts are not covering all categories/
foreach($allcats as $allcat) { $sitecats[] = $allcat->term_id; }; //get the category IDs

$cats = array_merge($cats, $sitecats); //add to the already collected categories
$cats = array_unique($cats); //only keep unique category IDs

$section_cats = array(1,23,48,128,9,19,67); //allowed categories for the automated section

$cats = array_intersect(  $cats, $section_cats );