Tips and tricks

$locations = get_theme_mod( 'nav_menu_locations' );

if(!empty($locations))
{
   foreach($locations as $locationId => $menuValue)
   {


      if($locationId=='primary'){

         if(isset($menu_id))
         {
            $locations[$locationId] = $menu_id;
            set_theme_mod('nav_menu_locations', $locations);
         }
      }

   }

}