Filter WordPress menu

// Filter wp_nav_menu() to add additional links and other output
function new_nav_menu_items($items) {
// gets passed a string of <li>’s
$homelink = ‘<li><a href=”‘ . home_url( ‘/’ ) . ‘”>’ . __(‘Home’) . ‘</a></li>’;
$items = $homelink . $items;
return $items;
}

add_filter( ‘wp_nav_menu_items’, ‘new_nav_menu_items’ );

Another Example

function new_nav_menu_items($items) {
$cat_menu_list = wp_list_categories( ‘echo=0&title_li=&child_of=124′ );
$items = $items . $cat_menu_list;
return $items;
}
add_filter( ‘wp_nav_menu_items’, ‘new_nav_menu_items’ );