WordPress Previous and Next posts with the same taxonomy term

get_adjacent_custom_posts.php

<?php
/**
 * Retrieve the adjacent posts for a specific custom post type and taxonomy.
 *
 * This function returns the previous and next posts based on the provided
 * custom post type, taxonomy, and ordering by published date in descending order (newest first).
 *
 * @param int    $post_id   The ID of the current post.
 * @param string $taxonomy  The taxonomy to filter posts by.
 * @param string $post_type The custom post type to query.
 *
 * @return array An associative array with 'previous' and 'next' keys containing post objects or null if not available.
 *
 * @example
 * $adjacent_posts = get_adjacent_custom_posts(123, 'custom_taxonomy', 'custom_post_type');
 * if ($adjacent_posts['previous']) {
 *     echo 'Previous Post: ' . $adjacent_posts['previous']->post_title;
 * }
 * if ($adjacent_posts['next']) {
 *     echo 'Next Post: ' . $adjacent_posts['next']->post_title;
 * }
 */
function get_adjacent_custom_posts($post_id, $taxonomy, $post_type) {
    $current_post = get_post($post_id);

    if (!$current_post) {
        return [
            'previous' => null,
            'next' => null
        ];
    }

    $args = [
        'post_type'      => $post_type,
        'posts_per_page' => -1,
        'tax_query'      => [
            [
                'taxonomy' => $taxonomy,
                'field'    => 'term_id',
                'terms'    => wp_get_post_terms($post_id, $taxonomy, ['fields' => 'ids']),
            ],
        ],
        'orderby'        => 'date',
        'order'          => 'DESC',
        'fields'         => 'ids',
    ];

    $posts = get_posts($args);

    if (!$posts) {
        return [
            'previous' => null,
            'next' => null
        ];
    }

    $current_index = array_search($post_id, $posts);

    $previous_post_id = $current_index !== false && $current_index > 0 ? $posts[$current_index - 1] : null;
    $next_post_id = $current_index !== false && $current_index < count($posts) - 1 ? $posts[$current_index + 1] : null;

    $previous_post = $previous_post_id ? get_post($previous_post_id) : null;
    $next_post = $next_post_id ? get_post($next_post_id) : null;

    return [
        'previous' => $previous_post,
        'next' => $next_post,
    ];
}