<?php
class BB_Nav_Footer_Walker extends Walker_Nav_Menu {
    function start_lvl(&$output, $depth = 0, $args = array()) {
        $output .= "\n<p class=\"sub-menu\">\n";
    }

    function end_lvl(&$output, $depth = 0, $args = array()) {
        $output .= "\n</p>\n";
    }
}
?>