next-prev-cpt-parent

function jeg_next_prev_portfolio($parentid, $currentid, $to, $category = '')
    {
        $portfolioquery = array(
            'post_type' => 'portfolio',
            'meta_query' => array(
                array(
                    'key' => 'portfolio_parent',
                    'value' => array($parentid),
                    'compare' => 'IN',
                )
            ),
            'orderby' => 'menu_order',
            'order' => 'ASC',
            'nopaging' => true
        );


        if($category !== '') {
            $portfolioquery['tax_query'] =
                array(
                    array(
                        'taxonomy'  =>  'portfolio_category',
                        'terms'     =>  $category,
                        'field'     => 'id',
                        'operator'  => 'IN'
                    )
                );
        }

        $query = new WP_Query($portfolioquery);

        $result = $query->posts;
        $currentpost = 0;

        foreach($result as $key => $res) {
            if($currentid === $res->ID) {
                $currentpost = $key;
                break;
            }
        }

        if($to === 'next') {
            $nextpost = $currentpost + 1;
            if($nextpost >= sizeof($result)) {
                $nextpost = 0;
            }

            $nextcontent = $result[$nextpost];
            return $nextcontent->ID;
        } else {
            $prevpost = $currentpost - 1;
            if($prevpost < 0) {
                $prevpost = sizeof($result) - 1;
            }
            $prevcontent = $result[$prevpost];
            return $prevcontent->ID;
        }
    }