pid, posterid, content
1 , 26 , blabla
2 , 8 , sec balnla
3 , 9 , lollll
4 , 26 , orddddd
5 , 10 , sssssdsd
...

function getComments($poster){
    $sql = "SELECT * FROM table WHERE posterid = $poster"
    $smt = $db->query($sql);
    $total = $smt->rowCount();

    $result = $db->prepare($sql . " ORDER BY pid DESC $limit");
    $result->execute();
    while ($row = $result->fetch(PDO::FETCH_ASSOC)) {
        $Comments[] = $row;
    }
    return [
        'comments' => $Comments,
        'total' => $total
    ];
}

$poster = 26;
$pid = 18;
$posttperpage = 10;

public function getPostPage($pid, $poster, $posttperpage)
{
    $page = '';

    $totlaPosts = getComments($poster)['total']; //70

    $pages = $totlaPosts / $posttperpage; //7

    for($i=0, $i<$totlaPosts, $i--)
    {
        // ?????
        // i confuzed no idead what can i do :(

    }
    return $page; //it must be 2
}

$k = 1;
for($i=0, $i<$pid; $i += $posttperpage){
    $page = $k;
    $k +=1;
}

//after loop $page now contains what page the post is on.