wp_adjacent_posts_links.php

<?php
function tb_get_previous_post_link(){
    global $post;
    $prev_link = "";
    $prev_post = get_previous_post();
    if($prev_post){
        $prev_link = get_the_permalink($prev_post->ID);
    }
    return $prev_link;
}

function tb_get_next_post_link(){
    global $post;
    $next_link = "";
    $next_post = get_next_post();
    if($next_post){
        $next_link = get_the_permalink($next_post->ID);
    }
    return $next_link;
}