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Using wp_nav_menu() , how can I append a value at end of each URL?
For example, I have the following url: http://www.example.com/ but I have to append a language parameter at the end, so the url should be: http://www.example.com/?lang=$language

2

I found the solution modifying the behaviour of wp_nav_menu with the wp_get_nav_menu_items-filter. Here's a somewhat complete example:

class ModifyLinkFilter {
protected $_prio = 10;
protected $_args;

public function __construct($addargs = array(), $prio = 10) {
    $this->_args = $addargs;
    $this->_prio = $prio;

    if(!empty($addargs)) {
        $this->register();
    }
}

public function register() {
    add_filter('wp_get_nav_menu_items',
        array($this, 'on_nav_items'), $this->_prio, 3);
}

public function unregister() {
    remove_filter('wp_get_nav_menu_items',
        array($this, 'on_nav_items'), $this->_prio, 3);
}

public function on_nav_items($items, $menu, $args) {
    foreach($items as $item) {
        if(!empty($item->url)) {
            $item->url = self::modifyUrlSimple($item->url, $this->_args);
        }
    }
    return $items;
}

public static function modifyUrlSimple($url, $query) {
    $url .= strchr($url, '?') === false ? '?' : '&';
    $url .= http_build_query($query);
    return $url;
}
}


// You can use the class like that
$language = "de";
$args = array('lang' => $language, 'foo' => 'bar');
$linkfilter = new ModifyLinkFilter($args); 
wp_nav_menu();
$linkfilter->unregister();    

This modifies every item in the navigation menu. So if you have an external link, it will be changed as well.

Plus, modifying the URL is not as easy as it seems. The URL of an item can be /blabla?myvalue=5#anchor which would look like /blabla?myvalue=5#anchor&lang=de&foo=bar after the modification.

1

You can either use a whole custom Walker - see codex or just a filter call to walker_nav_menu_start_el to regenerate link.

In case of filter, your code in theme's functions.php would look like this:

add_filter( 'walker_nav_menu_start_el', 'my_language_link_function', 10, 4 );

function my_language_link_function( $item_output, $item, $depth, $args ){
    $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

    $class_names = $value = '';

    $classes = empty( $item->classes ) ? array() : (array) $item->classes;
    $classes[] = 'menu-item-' . $item->ID;

    $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
    $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

    $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
    $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

    $output .= $indent . '<li' . $id . $value . $class_names .'>';

    $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
    $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
    $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
    //Your code generationg lang parameter goes here:
    $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

    $item_output = $args->before;
    $item_output .= '<a'. $attributes .'>';
    $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
    $item_output .= '</a>';
    $item_output .= $args->after;
    return $item_output;
}

This code is a copy of start_el function of Walker_Nav_Menu class, which you can find in wp-includes/nav-menu-template.php - see it's code.

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