modify menu item links dynamically

on our wordpress site we use utm_source in the querystring to determine the campaign that brought a customer to our site.

I need to dynamically add the passed utm_source from the initial page to all the menu items on the site. How can I modify all links in the wordpress menus to add the querystring variables?

Answers:

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Method 1

Can be changed using filter

add_filter('wp_get_nav_menu_items', 'add_utm_to_links', 10, 3);

function add_utm_to_links($items, $menu, $args) {

    foreach($items as $item) {
        if(!empty($item->url)) {
            $item->url .=  strchr($url, '?') === false ? '?' : '&';  
            $item->url .= 'utm=value';
        }
    }

    return $items;
}

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