Let’s take a look at this structure:

name, url, ….., level.
level is an integer that represents the level of a node in tree.
here is an example:

Programming Languages, example.com/languages, 0
PHP, example.com/languages/php, 1
Manual, example.com/languages/php/manual, 2
Examples, example.com/languages/php/examples, 2
Java, example.com/languages/java, 1
Python, example.com/languages/python, 1

and simple php function:
define(”TAB”, “    ”);
function displayTree($tree) {
while list($key, $val) = each($tree) {
echo str_repeat(TAB, $val['level']).’‘.$val['name'].’‘;
}
}

In this way you have N-deep tree with single loop. Of course your tree array must be sorted first. You can get similar array from database using nested sets.
Here is more info about nested sets:
http://dev.mysql.com/tech-resources/articles/hierarchical-data.html

With this solution adding or editing new rows is heavier compared to standard way but selecting is very quick. For categories is perfect because categories are not changing each day :)