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<?php
function pn_body_class_add_categories( $classes ) {
		
	// Only proceed if we're on a single post page
	if ( !is_single() )
		return $classes;
	
	// Get the categories that are assigned to this post
	$post_categories = get_the_category();
		
	// Loop over each category in the $categories array
	foreach( $post_categories as $current_category ) {
		
		// Add the current category's slug to the $body_classes array
		$classes[] = 'category-' . $current_category->slug;
		
	}
	
	// Finally, return the $body_classes array
	return $classes;
}
add_filter( 'body_class', 'pn_body_class_add_categories' );