Getting Next/Previous Post ID using Current Post ID in Wordpress - php

Question

I want to write a custom next/prev function to dynamically display post information in a fancybox pop-up. So I need to use PHP to get the next and previous Post ID based on whatever Post is currently showing. I know what the current post ID is and I can send that to a function ok, but I can't figure out how to use that ID to obtain the adjacent IDs.
Edit:Here is my code so far (which is not working)
<?php
require_once("../../../wp-blog-header.php");
if (isset($_POST['data'])){
$post_id = $_POST['data'];
}else{
$post_id = "";
}
$wp_query->is_single = true;
$this_post = get_post($post_id);
$in_same_cat = false;
$excluded_categories = '';
$previous = false;
$next_post = get_adjacent_post($in_same_cat,$excluded_categories,$previous);
$post_id = $next_post->id;
$title = $next_post->post_title;
$dataset = array ( "postid"=>$post_id, "posttitle"=>$title );
//Because we want to use json, we have to place things in an array and encode it for json.
//This will give us a nice javascript object on the front side.
echo json_encode($dataset);
?>

Answer 1

get_adjacent_post() uses the global $post as its reference point, so you'll want to replace this:
$this_post = get_post($post_id);
with this:
global $post;
$post = get_post($post_id);
WordPress also provides get_next_post() and get_previous_post(), which you can use here instead of using get_adjacent_post() with all of those arguments. Here's the final product:
<?php
require_once("../../../wp-blog-header.php");
if (isset($_POST['data'])){
$post_id = $_POST['data'];
}else{
$post_id = "";
}
$wp_query->is_single = true;
global $post;
$post = get_post($post_id);
$previous_post = get_previous_post();
$next_post = get_next_post();
$post_id = $next_post->id;
$title = $next_post->post_title;
$dataset = array ( "postid"=>$post_id, "posttitle"=>$title );
//Because we want to use json, we have to place things in an array and encode it for json.
//This will give us a nice javascript object on the front side.
echo json_encode($dataset);
?>
I'm not sure what keys you'd like to use for the IDs and titles of the previous and next posts in the $dataset array, so I'll leave that as is for now.

Answer 2

To get this to work I had to change $next_post->id; to $next_post->ID;, i.e. capitalise ID.
I used it in Wordpress on index.php like this – I inserted this at the top of the loop:
<div class="work-post" id="<?php the_ID(); ?>">
then within the loop I use this:
`
if (isset($_POST['data'])){
$post_id = $_POST['data'];
}else{
$post_id = "";
}
$wp_query->is_single = true;
global $post;
$post = get_post($post_id);
$next_post = get_next_post();
$previous_post = get_previous_post();
?>
<!-- call only if a value exists -->
<?php if($next_post) : ?>
<div>PREV.</div>
<?php endif; ?>
<!-- call only if a value exists -->
<!-- call only if a value exists -->
<?php if($previous_post) : ?>
<div>NEXT</div>
<?php endif; ?>
<!-- call only if a value exists -->`

Answer 3

This is my code, it worked good. $post_id is current post ID.
function get_previous_post_id( $post_id ) {
// Get a global post reference since get_adjacent_post() references it
global $post;
// Store the existing post object for later so we don't lose it
$oldGlobal = $post;
// Get the post object for the specified post and place it in the global variable
$post = get_post( $post_id );
// Get the post object for the previous post
$previous_post = get_previous_post();
// Reset our global object
$post = $oldGlobal;
if ( '' == $previous_post )
return 0;
return $previous_post->ID;
}
function get_next_post_id( $post_id ) {
// Get a global post reference since get_adjacent_post() references it
global $post;
// Store the existing post object for later so we don't lose it
$oldGlobal = $post;
// Get the post object for the specified post and place it in the global variable
$post = get_post( $post_id );
// Get the post object for the next post
$next_post = get_next_post();
// Reset our global object
$post = $oldGlobal;
if ( '' == $next_post )
return 0;
return $next_post->ID;
}
Then you just call function. Example:
echo get_previous_post_id( $current_id );
echo get_next_post_id( $current_id );
Good luck!