Custom Walker: how to get ID in function start_lvl

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I am making my first custom walker to build an accordion menu. To start with I used this example: http://bitacre.com/2025/custom-nav-menu-walker-for-wordpress-themes

There are two functions. First start_lvl and then start_el.

In start_el the ID get’s implemented by $item->ID. Does anybody know how I can do this in start_lvl as well? I need to give the (surrounding lower level navigation) an ID so I can trigger it to collapse in the accordion menu.

What I’m trying to generate is something like this:

<a href="#collapse2">Titel 2</a>
<ul id="collapse2">Lower Level Menu 2</ul>
<a href="#collapse3">Titel 3</a>
<ul id="collapse3">Lower Level Menu  3</ul>

My code for the start_lvl function:

// add id's and classes to ul sub-menus
function start_lvl( &$output, $depth, $item ) {
    // depth dependent classes
    $indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' ); // code indent
    $display_depth = ( $depth + 1); // because it counts the first submenu as 0
    $pgid = ; // How to get ID in here??
    $classes = array(
        'sub-menu',
        ( $display_depth == 1  ? 'accordion-body collapse' : '' ),
        ( $display_depth % 2  ? 'menu-odd' : 'menu-even' ),
        ( $display_depth >=2 ? 'sub-sub-menu' : '' ),
        'menu-depth-' . $display_depth
        );
    $ids = array(
        'collapse' . $pgid
        );
    $class_names = implode( ' ', $classes );
    $id_name = implode( ' ', $ids );

    // build html
    $output .= "\n" . $indent . '<ul id="' . $id_name . '" class="' . $class_names . '">' . "\n";
}

3

I just had to do this in one of my themes… Since you don’t have access to the $item variable at that stage of the Walker, you would want to store your current item in a more global scope at the moment you do have access to it. The following code will make more sense… note: I’ve stripped everything but the relevant code. 

class ThemeTruck_Nav_Walker extends Walker_Nav_Menu {
   private $curItem;

  // retrieve the curItem
  function start_lvl(&$output, $depth = 0, $args = array()) {
    var_dump($this->curItem );
  }

  // store the curItem
  function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
    $this->curItem = $item;
  }

 }

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