In the loop use <?php the_category(', '); ?>
or outside the loop :
$post_categories = wp_get_post_categories( $post_id );
foreach($post_categories as $c){
$cat = get_category( $c );
echo $cat->name . ",";
}
In the loop use <?php the_category(', '); ?>
or outside the loop :
$post_categories = wp_get_post_categories( $post_id );
foreach($post_categories as $c){
$cat = get_category( $c );
echo $cat->name . ",";
}