Functions that returns either next or prev post id or the url. Infinte Wordpress Posts Loop.

pre-next-links.php

/* ---------------------------------------------------------------------------
 * INFINITE PREV-NEXT POSTS LINKS (o id)
 * --------------------------------------------------------------------------- */

  function prevPostLink() {
    if( get_adjacent_post(false, '', false) ) { 
      $postLink = get_permalink(get_adjacent_post(false,'',false));
    } else { 
      $first = new WP_Query('posts_per_page=1&order=DESC'); $first->the_post();
      $postLink = get_permalink();
      wp_reset_query();
    }; 
    return $postLink;
  }
   
  function nextPostLink() {     
    if( get_adjacent_post(false, '', true) ) { 
      $postLink = get_permalink(get_adjacent_post(false,'',true));
    } else { 
      $last = new WP_Query('posts_per_page=1&order=ASC'); $last->the_post();
      $postLink = get_permalink();
      wp_reset_query();
    }; 
    return $postLink;
  }

  function prevPostId() {
    if( get_adjacent_post(false, '', false) ) { 
      $post = get_adjacent_post(false,'',false);
    } else { 
      $first = new WP_Query('posts_per_page=1&order=DESC'); $first->the_post();
      $post = get_post();
      wp_reset_query();
    }; 
    return $post->ID;
  }
   
  function nextPostId() {     
    if( get_adjacent_post(false, '', true) ) { 
      $post = get_adjacent_post(false,'',true);
    } else { 
      $last = new WP_Query('posts_per_page=1&order=ASC'); $last->the_post();
      $post = get_post();
      wp_reset_query();
    }; 
    return $post->ID;
  }

Based -> https://gist.github.com/banago/5603826 (full prev-next post wordpress function)