Return either an array with all categories name from a post or a formatted string that bay be echoed.

Wordpress post category listing

function get_categories_name($post_id, $formated = false, $separator = " + "){
	$categories 		= wp_get_post_categories($post_id);
	$categories_name 	= array();
	$formatted_string 	= "";

	foreach ($categories as $category){
		array_push($categories_name, get_category($category)->name);
	}

	if (!$formated)
		return $categories_name;
	
	foreach ($categories_name as $key => $name){
		$formatted_string .= $name;

		if ($key < count($categories_name) - 1)
			$formatted_string .= $separator;
	}

	return $formatted_string;
}