How to target specific wp_nav_menu in function?

Question

I am adding a specific class on my wp_nav_menu via function.php but I am not able to target a specific menu: This is what I got

function add_menuclass_active($ulclass) {
   return preg_replace('/<a /', '<a class="list-group"', $ulclass, 1);
}
 add_filter('wp_nav_menu', 'add_menuclass_active');

I tried also using with no luck

function add_menuclass_active($ulclass) {
  if( $ulclass['theme_location'] == 'CUSTOM MENU' )
   return preg_replace('/<a /', '<a class="list-group"', $ulclass, 1);
}
add_filter('wp_nav_menu', 'add_menuclass_active');

Answer 1

Thanks to an answer from wordpress stack I got this solution:

add this to your functions.php

register_nav_menus(array(
'top-menu' => __('Menu1', 'twentyfourteen'),
'side-menu' => __('Menu2', 'twentyfourteen'),
'footer-menu' => __('Menu3', 'twentyfourteen')
)
);

function my_walker_nav_menu_start_el($item_output, $item, $depth, $args) {
    $menu_locations = get_nav_menu_locations();

    if ( has_term($menu_locations['top-menu'], 'nav_menu', $item) ) {
       $item_output = preg_replace('/<a /', '<a class="list-group" ', $item_output, 1);
    }

    return $item_output;
}
add_filter('walker_nav_menu_start_el', 'my_walker_nav_menu_start_el', 10, 4);

at last you must select the option "Menu1" for the specific menu on which you have to add the anchor custom classes from dashboard Apperance->menus. [select menu2 or menu3 for other menus whose anchor links does not need the custom-class]

To add "active class" to the first menu item of the particular menu then try this one:

function my_walker_nav_menu_start_el($item_output, $item, $depth, $args) {
    $menu_locations = get_nav_menu_locations();


    if ( has_term($menu_locations['top-menu'], 'nav_menu', $item) ) {
       $item_output = preg_replace('/<a /', '<a class="list-group" ', $item_output, 1);
if ($item->menu_order == 1){
 $item_output = preg_replace('/<a /', '<a class="list-group active" ', $item_output, 1);
}
    }

    return $item_output;
}
add_filter('walker_nav_menu_start_el', 'my_walker_nav_menu_start_el', 10, 4);

if the active class must be added to the first menu item of all menus then use this:

function my_walker_nav_menu_start_el($item_output, $item, $depth, $args) {
    $menu_locations = get_nav_menu_locations();


    if ( has_term($menu_locations['top-menu'], 'nav_menu', $item) ) {
       $item_output = preg_replace('/<a /', '<a class="list-group" ', $item_output, 1);
    }
 if ($item->menu_order == 1){
 $item_output = preg_replace('/<a /', '<a class="active" ', $item_output, 1);
}
    return $item_output;
}
add_filter('walker_nav_menu_start_el', 'my_walker_nav_menu_start_el', 10, 4);

Answer 2

Theme location is stored in the nav menu arguments, you need to add it in your function parameters:

function add_menuclass_active( $nav_menu, $args ) {
    if( $args->theme_location == 'CUSTOM MENU' )
        return preg_replace( '/<a /', '<a class="list-group"', $nav_menu, 1 );
    return $nav_menu;
}
add_filter( 'wp_nav_menu', 'add_menuclass_active', 10, 2 );