Unable to go to next and previous record in php

             if ($replies_count !== 0) {
// find out how many rows are in the table 
$sql = "SELECT COUNT(*) FROM `phpc_topic_replies` WHERE `phpc_topic_serial`=$topic_serial";                     
$result = mysqli_query($coni,$sql) or trigger_error("SQL", E_USER_ERROR);
$r = mysqli_fetch_row($result);
$numrows = $r[0];
// number of rows to show per page
$rowsperpage = 15;
// find out total pages
$totalpages = ceil($numrows / $rowsperpage);

// get the current page or set a default
if (isset($_GET['currentpage']) && is_numeric($_GET['currentpage'])) {
   // cast var as int
   $currentpage = (int) $_GET['currentpage'];
} else {
   // default page num
   $currentpage = 1;
} // end if

// if current page is greater than total pages...
if ($currentpage > $totalpages) {
   // set current page to last page
   $currentpage = $totalpages;
} // end if
// if current page is less than first page...
if ($currentpage < 1) {
   // set current page to first page
   $currentpage = 2;
} // end if

// the offset of the list, based on current page 
$offset = ($currentpage - 1) * $rowsperpage;

// get the info from the db 
$get_replies = "SELECT * FROM `phpc_topic_replies` WHERE `phpc_topic_serial`=$topic_serial AND `phpc_reply_valid`='yes' order by phpc_reply_serial asc LIMIT $offset, $rowsperpage ";                   
//$sql = "SELECT location, caption FROM photos order by id desc LIMIT $offset, $rowsperpage";
$result = mysqli_query($coni,$get_replies);
// while there are rows to be fetched...
while ($rep = mysqli_fetch_assoc($result)) {
$reply_serial = $rep['phpc_reply_serial'];
?>

<div>
<h4>echo whatevr is gonna be paginted here like this "?php echo new ??</h4>

</div>

 <?php
             
                                        }


$range = 2;
    echo '<div class="row"> <h3 class="center"> Read more replies </h3> <ul style="text-align: center;" class="pagination">';
if ($currentpage > 1) {

   echo "<li> <a href='$main_url/read/$topic_url/1'>First Page</a></li> ";

   $prevpage = $currentpage - 1;

   echo "<li> <a href='$main_url/read/$topic_url/$prevpage'><</a> </li>";
}
for ($x = ($currentpage - $range); $x < (($currentpage + $range) + 1); $x++) {
  if (($x > 0) && ($x <= $totalpages)) {
     if ($x == $currentpage) {
        echo "<li> <a href='#' class='active'>$x</a>";
  } else {
        echo "<li> <a href='$main_url/read/$topic_url/$x'>$x</a> </li> ";
      } // end else
   } // end if 
} // end for
                 
// if not on last page, show forward and last page links        
if ($currentpage != $totalpages) {
   // get next page
   $nextpage = $currentpage + 1;
    // echo forward link for next page 
   echo "<li> <a href='$main_url/read/$topic_url/$nextpage'>></a></li> ";
   // echo forward link for lastpage
   echo "<li> <a href='$main_url/read/$topic_url/$totalpages'>Last Page</a></li> ";
  
} // end if
             }
              echo '</ul></div>';
?>