Let me explain my situation, I have left menu and menu name is a category slug. Now I want to add next and previous button at the bottom of every post which refer the next and previous menu item. here is my code:

 * 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' ) );

 foreach ( $menuitems as $item ):
    if($item->url =='#')


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


   if ($id==$post->ID){


 if (empty($nextid)){
 if (empty($previd)){

 $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;

This code is working while I keep it inside any template file like 'index.php' but when I create a simple plugin with same code it is not working. To explore the bug I try to print category object. It prints empty array. Whats wrong is going on my code? Or Is it not possible to use this code inside a plugin instead of template file? thanks for your time.

  • Why is most of your code outside of your add_next_prev_btn function?
    – Milo
    Jul 22, 2018 at 15:34

1 Answer 1


Your code will run when the plugin is first initiated, which will be before the post data has been set up. Move all of your code from $cat = get_the_category($id); up to (but not including) add_filter... inside the function.

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