<?php
/**
 * Plugin Name:   Next Previous button bootstrap
 * Description:   Super simple plugin to add next and Previous button
 * Version:       1.0.0
 * Author:        Abdus Sattar Bhuiyan
 * Author URI:   https://www.facebook.com/abdus.s.bhuiyan
 */

$cat = get_the_category($id);
print_r($cat); exit;
$cat = $cat[0];
$menu_slug = strtolower($cat->name);

$menuitems = wp_get_nav_menu_items( $menu_slug, array( 'order' => 'DESC' ) );

 $i=-1;
 foreach ( $menuitems as $item ):
    if($item->url =='#')
      continue;

     $i++;

   $id = get_post_meta( $item->ID, '_menu_item_object_id', true );
   $page = get_page( $id );
   $link = get_page_link( $id );

     $linkarray.=$id.",";
     $urlarray.=$link.",";

   if ($id==$post->ID){
     $previd=$i-1;
     $nextid=$i+1;
   }
 endforeach;

 $linkarray=explode(',',$linkarray);
 $urlarray=explode(',',$urlarray);

 $nextid=$urlarray[$nextid];
 if (empty($nextid)){
     $nextid=$urlarray[0];
 }
 $previd=$urlarray[$previd];
 if (empty($previd)){
     $previd=$urlarray[$i];
 }

 $next_prev_btn = '<a href="<?php echo $nextid; ?>">Next Item</a>';
 $next_prev_btn .= '<a href="<?php echo $previd; ?>">Previous Item</a>';


 add_filter( 'the_content', 'add_next_prev_btn' );

 function add_next_prev_btn( $content ) {
         return $content . $next_prev_btn;
 }