จะรับค่า จาก dropdown list เป็นตัวแปร session อย่างไรค่ะ

<select name="customer_id" id="customer_id" class="form-control" onchange="findcustomer_current(this.value)">
                                       <option value="">
                                                --กรุณาเลือก--
                                            </option>
<?php
$sqld=" select * from tb_customer";
if(isset($_GET['customer_id']))
        {
             $customer_id=$_GET['customer_id'];
             $sqld.=" where";
             $sqld.=" customer_id='$customer_id'";
        }
             $resultd=$cls_con->select_base($sqld);
             while($rowd=mysqli_fetch_array($resultd))
        { 
             $customer_id=$rowd['customer_id'];
             $customer_fname=$rowd['customer_fname'];
 ?>
             <option value="<?=$customer_id;?>">
             <?=$customer_fname;?>
             </option>
             <?php } ?>
</select>