How to do pagination?

Question

PHP

I have a database where projects are located, and there is navigation through these projects on the page using two buttons "Forward" and "Back"

<?php
$nextpost = next_post($post_id);
$prevpost = prev_post($post_id);
?>

<div>
<a href="/project?post_id=<?= $prevpost['id'] ?>"><button>Prev</button></a>
<a href="/project?post_id=<?= $nextpost['id'] ?>"><button">Next</button></a>
</div>

function next_post($post_id)
{
    global $link;
    $sql = "SELECT * FROM projects WHERE id > " . $post_id . " ORDER BY id ASC LIMIT 1";
    $result = mysqli_query($link, $sql);
    $nextpost = mysqli_fetch_assoc($result);
    return $nextpost;
}

function prev_post($post_id)
{
    global $link;
    $sql = "SELECT * FROM projects WHERE id < " . $post_id . " ORDER BY id DESC LIMIT 1";
    $result = mysqli_query($link, $sql);
    $prevpost = mysqli_fetch_assoc($result);
    return $prevpost;
}

The problem is that if, for example, I am on a project with id=5 (and id=6 does not yet exist) and at the same time I click on the "Forward" button, then I am transferred to a page with id=6, which does not exist and i just get an error.

How to make it so that when I click on the "Next" button, provided that I do not have the next project, I will be thrown to the page with the id of the very first project in the database. (Same with the back button)

Answer 1

It's not clear from the code below what happens.

if, for example, I am on a project with id=5 (and id=6 does not yet exist) and at the same time I click on the "Forward" button, then I am transferred to a page with id=6, which does not exist and I just get an error.

The general idea of ​​pagination: see what data you have in the database, based on this you form links to possible pages.
If you want to make it cyclic - well, do it, add a check "id greater than the current one is not found" to your logic and process it correctly.

Answer 2

Gods of clean code forgive me.

/**
 * Следующий пост
 *
 * @param $post_id
 *
 * @return string[]|null
 */
function next_post($post_id)
{
    global $link;
    $sql      = "SELECT * FROM projects WHERE id > " . $post_id . " ORDER BY id ASC LIMIT 1";
    $result   = mysqli_query($link, $sql);
    $nextpost = mysqli_fetch_assoc($result);

    if (!$nextpost) {
        return first_post();
    }

    return $nextpost;
}

/**
 * Предыдущий пост
 *
 * @param $post_id
 *
 * @return string[]|null
 */
function prev_post($post_id)
{
    global $link;
    $sql      = "SELECT * FROM projects WHERE id < " . $post_id . " ORDER BY id DESC LIMIT 1";
    $result   = mysqli_query($link, $sql);
    $prevpost = mysqli_fetch_assoc($result);

    if (!$prevpost) {
        return last_post();
    }

    return $prevpost;
}

/**
 * Первый пост
 *
 * @return string[]|null
 */
function first_post()
{
    global $link;
    $sql    = "SELECT * FROM projects WHERE ORDER BY id ASC LIMIT 1";
    $result = mysqli_query($link, $sql);
    $post   = mysqli_fetch_assoc($result);

    return $post;
}

/**
 * Последний пост
 *
 * @return string[]|null
 */
function last_post()
{
    global $link;
    $sql    = "SELECT * FROM projects WHERE ORDER BY id DESC LIMIT 1";
    $result = mysqli_query($link, $sql);
    $post   = mysqli_fetch_assoc($result);

    return $post;
}