Walker for Custom Post Type

walker.php

<?
class Custom_Display_Children_Walker extends Walker_Nav_Menu {

	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        global $wp_query;

        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = '';

        $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>';

        $atts = array();
        $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
        $atts['target'] = ! empty( $item->target )     ? $item->target     : '';
        $atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
        $atts['href']   = ! empty( $item->url )        ? $item->url        : '';

        $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );

        $attributes = '';
        foreach ( $atts as $attr => $value ) {
                if ( ! empty( $value ) ) {
                        $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                        $attributes .= ' ' . $attr . '="' . $value . '"';
                }
        }

        $item_output = $args->before;
        $item_output .= '<a'. $attributes . $class_names .'>';
        /** This filter is documented in wp-includes/post-template.php */
        if (strpos($item->classes[0],'icon') !== false) {
        	$item_output .= cnp_isvg('icon-name='. $item->classes[0] .'&echo=0');
        }

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

        if ( $item->type == 'post_type_archive' && $item->title == 'Case Studies' ) {

        	$post_type_entries = get_posts( 'post_type=custom-post-type&numberposts=-1&orderby=menu_order&order=ASC' );

        	if (empty($post_type_entries))
        		return;

        	$children = '';
        	foreach ($post_type_entries as $key => $entry) {
        		$class_names = '';
        		if ($wp_query->query_vars['name'] == $entry->post_name)
        			$class_names .= 'class="current-menu-item"';

        		$children .= '<li><a '. $class_names .' href="'. get_permalink($entry->ID) .'" >'. $entry->post_title .'</a></li>';
        	}

        	if (empty($children))
        		return;

        	$item_output .= '<ul class="children">'. $children .'</ul>';
        }

        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
}
}