<form action="sales_order.php" method="post"> //this form is for filtering the table by date
    <div class="input-group">
        <input type="date" class="form-control" required="required" name="valueToSearch" placeholder="Salesman Name">
        <div class="input-group-btn">
            <button class="btn btn-default" type="submit" name="search">
                <i class="glyphicon glyphicon-search"></i>
            </button>
        </div>
    </div>
</form>
<br />
<table class="table" id="customers">
    <thead>
        <tr>
            <th>Purchase ID</th>
            <th>Purchase Date</th>
            <th>Delivery Date</th>
            <th>Customer Name</th>
            <th>Address</th>
            <th>Salesman</th>
            <th>Amount</th>
            <th colspan="2">Action</th>
            <th>Status</th>
            <th>Remarks</th>
        </tr>
    </thead>
    <tbody>
        <?php while($r = mysqli_fetch_array($search_result)): ?>
            <tr>
                <td><?php echo $r['so_id'] ?></td>
                <td><?php echo $r['so_date'] ?></td>
                <td><?php echo $r['del_date'] ?></td>
                <td><?php echo $r['so_customer'] ?></td>
                <td><?php echo $r['so_address'] ?></td>
                <td><?php echo $r['so_salesman'] ?></td>
                <td><?php echo number_format ($r['so_amount'],2) ?></td>
                <td><a href="#" onclick='Approve(<?php echo $r['so_id'] ?>)'>Approve</a></td>
                <td><a href="#" onclick='Decline(<?php echo $r['so_id'] ?>)'>Decline</a></td>
                <td><?php echo $r['so_status'] ?></td>
                <td><?php echo $r['so_remarks'] ?></td>
            </tr>
       <?php endwhile; ?>
    </tbody>
</table>
<script>
function Approve(id) {
    if (confirm("Are you sure you want to approve?")) {
        // your time and date stamp
      window.location.href='add.php?id='+id;
    }
    return false;
}
</script>

<?php
 $databaseHost = "localhost";
 $databaseUser = "root";
 $databasePassword = "";
 $databaseName = "csl_otd";

 $con=mysql_connect($databaseHost ,$databaseUser ,$databasePassword )or die ('Connection Error');
  mysql_select_db("csl_otd",$con) or die ('Database Error');

if(isset($_GET['so_id']) && !empty($_GET['so_id'])){
    $sql = "UPDATE sales_order SET so_status='Approve', so_remarks = NOW() WHERE so_id =" . $_GET['id']  ;
    $resultadd = mysql_query($sql,$con);
}

<td><?php echo $r['so_id'] ?></td>
      <td><?php echo $r['so_date'] ?></td>
      <td><?php echo $r['del_date'] ?></td>
      <td><?php echo $r['so_customer'] ?></td>
      <td><?php echo $r['so_address'] ?></td>
      <td><?php echo $r['so_salesman'] ?></td>
      <td><?php echo number_format ($r['so_amount'],2) ?></td>
      <td><button type='submit' name='sAcc' value='{$sId}'>Approve</button></td>
      <td><button type='submit' name='sRej' value='{$sId}'>Decline</button></td>
      <td><?php echo $r['so_status'] ?></td>
      <td><?php echo $r['so_remarks'] ?></td>


    </tr>

<?php endwhile; ?>

<?php
 if (isset($_POST['sAcc']) && intval($_POST['sAcc'])) {
        $user_id = (int) $_POST['sAcc'];

        $sql = "UPDATE sales_order SET so_status='Approve', so_remarks = NOW() WHERE so_id = $sId"  ;


    }
    if (isset($_POST['sRej']) && intval($_POST['sRej'])) {
        $user_id = (int) $_POST['sRej'];


       $sql = "UPDATE sales_order SET so_status='Decline', so_remarks = NOW() WHERE so_id = $sId"  ;
    }



?>
  </tbody>

</table>

</div>