WordPress menu filter

wordpress_menu_filter.php

<?php

/** Show children only of current page */
add_filter( 'wp_nav_menu_objects', 'olab_nav_menu_objects_start_in', 10, 2 );
function olab_nav_menu_objects_start_in( $sorted_menu_items, $args ) {
    global $wp_query;
	switch ( $args->menu_id ) {

    	case 'sidebar-menu' : // change this to the ID of the menu you want to target
	        $current     = false;
	        $last_parent = false;
	        $menu        = array();

	        foreach( $sorted_menu_items as $item ) {
	            // The big daddy, final parent of the current menu item
	            // If we're already at the last item this will remain false
	            if ( $item->current_item_ancestor && $item->menu_item_parent == '0' )
	                $last_parent = $item;

	            if ( $item->current )
	                $current = $item;

	        }

	        // Only show on relevant pages
	        if ( $current ) {
	            $keys = array();
	            $last_parent = ( !$last_parent ) ? $current : $last_parent;

		    $children   = _olab_submenu_get_children_ids( $last_parent->ID, $sorted_menu_items );
		    $children[] = $last_parent->ID;

	            foreach ( $sorted_menu_items as $key => $item ) {

	                if ( ! in_array( $item->ID, $children ) )
	                    unset( $sorted_menu_items[$key] );

	            }
		    $first_item = array_shift( array_values( $sorted_menu_items ) );
		    $last_item  = array_pop( array_values( $sorted_menu_items ) );

	    	    $first_item->classes[] = 'first-child';
	            $last_item->classes[] = 'last-child';

	        } else {
                        // Empty the list if there's no parent visible
	            $sorted_menu_items = array();
	        }

	       	break;

	}
    return $sorted_menu_items;
}


/** Helper function for showing the correct menu tree */
function _olab_submenu_get_children_ids( $id, $items ) {
    $ids = wp_filter_object_list( $items, array( 'menu_item_parent' => $id ), 'AND', 'ID' );

    foreach ( $ids as $id )
        $ids = array_merge( $ids, _olab_submenu_get_children_ids( $id, $items ) );

    return $ids;
}