How to add html,css class="category-" to posts

I needed this and I mixed up some code with help of bcworkz
in this thread LINK
so thing is replace (around 270 line)
$output .= "<div class='pfpItem entry-content>";

with this:

$post_categories = wp_get_post_categories( $child->ID );
				$cats = array();

				foreach($post_categories as $c){
					$cat = get_category( $c );
					$cats[] = $cat->slug ;
				}

				$classes = implode ( " category-",  $cats );

			$output .= "<div class='pfpItem entry-content category-$classes'>";

http://ww.wp.xz.cn/extend/plugins/posts-for-page/