https://codepen.io/tayfunerbilen/pen/AXywjw

http://paperjs.org/ ile yapmışlar.

::-webkit-scrollbar {
    width: 10px;
}

::-webkit-scrollbar-track {
    background: transparent; 
}
 
::-webkit-scrollbar-thumb {
    border-radius: 5px;
    background: #888; 
}

::-webkit-scrollbar-thumb:hover {
    background: #555; 
}

$("#icerik ul li").on("click", function(){
    var id = $(this).attr("id");
    alert(id);
})

1.Sorunun cevabı @merchizm 'in de dediği gibi

function pagination($current_page, $total_pages, $page_url){
// bla bla bla
}

kısmını function.php ye atman yeterli.

2.Sorunuda anladığım kadarıyla mysql'deki JOIN fonksiyonuyla halledebilirsin.
https://www.db-fiddle.com/f/u7iaQy3VT7tFjtFMEi4uRg/0
https://www.w3schools.com/sql/sql_join_inner.asp
https://www.w3schools.com/sql/sql_join_left.asp

function pagination($current_page, $total_pages, $page_url)
{
    $pagination = '';
    if($total_pages > 0 && $total_pages != 1 && $current_page <= $total_pages){
        $pagination .= '<ul class="pagination">';

        $right_links    = $current_page + 3;
        $previous       = $current_page - 1;
        $next           = $current_page + 1;
        $first_link     = true;

        if($current_page > 1){
			$previous_link = ($previous==0)?1:$previous;
            $pagination .= '<li class="page-item"><a class="page-link" href="'.$page_url.'?sayfa=1">İlk</a></li>';
            $pagination .= '<li><a class="page-link" href="'.$page_url.'?sayfa='.$previous_link.'">Önceki</a></li>';
                for($i = ($current_page-2); $i < $current_page; $i++){
                    if($i > 0){
                        $pagination .= '<li><a class="page-link" href="'.$page_url.'?sayfa='.$i.'">'.$i.'</a></li>';
                    }
                }
            $first_link = false;
        }

        if($first_link){
            $pagination .= '<li class="page-item disabled"><a class="page-link">'.$current_page.'</a></li>';
        }elseif($current_page == $total_pages){
            $pagination .= '<li class="page-item disabled"><a class="page-link">'.$current_page.'</a></li>';
        }else{ //regular current link
            $pagination .= '<li class="page-item disabled"><a class="page-link">'.$current_page.'</a></li>';
        }

        for($i = $current_page+1; $i < $right_links ; $i++){
            if($i<=$total_pages){
                $pagination .= '<li class="page-item"><a class="page-link" href="'.$page_url.'?sayfa='.$i.'">'.$i.'</a></li>';
            }
        }
        if($current_page < $total_pages){
				$next_link = ($next > $total_pages)? $total_pages : $next;
                $pagination .= '<li class="page-item"><a class="page-link" href="'.$page_url.'?sayfa='.$next_link.'">İleri</a></li>';
                $pagination .= '<li class="page-item"><a class="page-link" href="'.$page_url.'?sayfa='.$total_pages.'">Son</a></li>';
        }

        $pagination .= '</ul>';
    }
    return $pagination;
}
$x = $db->prepare("select id from urun order by id DESC");
$x->execute();
$z = $x->fetchAll(PDO::FETCH_ASSOC);
$pageLength = 10;
$page = isset($_GET['sayfa']) ? $_GET['sayfa'] : 1;
$pages = ceil(count($z)/$pageLength);
if($page < 1) {
    $page = 1;
}
if($page > $pages){
    $page = $pages;
}
$start = ($page - 1) * $pageLength;
$dt = $db->prepare("select * from urun order by id DESC limit $start, $pageLength");
$dt->execute();
$data = $dt->fetchAll(PDO::FETCH_ASSOC);

foreach($data AS $item)
{
	//verileri yazdır
}

echo pagination($page, $pages, "http://localhost/index.php");

https://youtu.be/MQoXH9yNUyo

SELECT `yazilar`.*, GROUP_CONCAT(`kategoriler`.`k_adi`) AS KategoriAdi FROM `yazilar` INNER JOIN `kategoriler` ON FIND_IN_SET(`kategoriler`.`k_id`, `yazilar`.`kategori_id`) GROUP BY `yazilar`.`id`