<div id="post-nav" class="p-border">
    <?php
    $link = get_the_permalink();
    $slugs = parseCatPostURL($link);// тут просто выдираю название категории!  простейший метод на explode()
   
    $current_category = get_category_by_slug($slugs);
    

   
   
    
  
   
  $g=get_the_ID();
  $t=$current_category->term_id;
  $posts = $wpdb->get_results("SELECT  `ID`,`post_date`,`slug`, `post_title`,`post_date`, `post_content` 
FROM `wp_terms` AS `t1` 
INNER JOIN `wp_term_taxonomy` AS `t2` ON t1.term_id=t2.term_id 
INNER JOIN `wp_term_relationships`AS t3 ON t2.term_taxonomy_id=t3.term_taxonomy_id 
INNER JOIN `wp_posts` AS t4 ON t4.ID=t3.object_id 
INNER JOIN `wp_postmeta` AS t7 ON t4.ID=t7.post_id 
WHERE t2.taxonomy='category' AND t1.term_id={$t} AND t4.post_status='publish' AND t4.ID<{$g} and meta_value in (SELECT meta_value FROM `wp_postmeta` WHERE `post_id`={$g} and `meta_key`=\"_category_permalink\")
ORDER BY t4.post_date DESC
LIMIT 1");
    if (count($posts) > 0) {
        ?>
        <div class="post-previous tranz p-border">
            <a href="<?php echo the_permalink($posts[0]->ID); ?>
               " title="<?php echo $posts[0]->post_title ?>">
                <i class="fa fa-chevron-left" aria-hidden="true"></i>&nbsp;
                    <?php echo $posts[0]->post_title ?></a>
        </div> 

        <?php
    }else{
        ?>
            <div class="post-previous tranz p-border"> &nbsp</div> 
            <?php
    }
  
  
  
   
    
    $posts = $wpdb->get_results("SELECT  `ID`,`post_date`,`slug`, `post_title`,`post_date`, `post_content` 
FROM `wp_terms` AS `t1` 
INNER JOIN `wp_term_taxonomy` AS `t2` ON t1.term_id=t2.term_id 
INNER JOIN `wp_term_relationships`AS t3 ON t2.term_taxonomy_id=t3.term_taxonomy_id 
INNER JOIN `wp_posts` AS t4 ON t4.ID=t3.object_id 
INNER JOIN `wp_postmeta` AS t7 ON t4.ID=t7.post_id 
WHERE t2.taxonomy='category' AND t1.term_id={$t} AND t4.post_status='publish' AND t4.ID>{$g} and meta_value in (SELECT meta_value FROM `wp_postmeta` WHERE `post_id`={$g} and `meta_key`=\"_category_permalink\")
ORDER BY t4.post_date ASC
LIMIT 1");
    if (count($posts) > 0) {
        ?>
        <div class="post-next tranz p-border">
            <a href="<?php echo the_permalink($posts[0]->ID); ?>
               " title="<?php echo $posts[0]->post_title ?>">
                <?php echo $posts[0]->post_title ?> &nbsp;<i class="fa fa-chevron-right" aria-hidden="true"></i>&nbsp;
                    </a>
        </div> 

        <?php
    }else{
         ?>
            <div class="post-next tranz p-border"> &nbsp</div> 
            <?php
    }
  
  
  ?>
</div>