ค้นหาใน PHP Mysql จะตัดส่วนที่เป็น Drop down อย่างไรครับ

<?php
include('connect.php');
$cls_conn=new class_conn();
$id=$_REQUEST['id'];
$sql=" select * from tb_customer";
$sql.=" where customer_id='$id'";
$sql.=" limit 1";
 
$result=$cls_conn->select_base($sql);
while ($row = mysqli_fetch_array($result))
{
$customer_fname=$row['customer_fname'];
} 
 echo $customer_fname;
?>

<script> 
function findcustomer(str) 
    {                                
        if (str.length == 0) { 
        document.getElementById("customer_fname").innerHTML = "";
        return;
                             } else {
                                     var xmlhttp = new XMLHttpRequest();
                                     xmlhttp.onreadystatechange = function() {
                                         if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                                             document.getElementById("customer_fname").value = xmlhttp.responseText;                                               
                                            var qty= xmlhttp.responseText;                                              
                                         }
                                     }
                                     xmlhttp.open("GET", "find_customer.php?id="+str, true);
                                     xmlhttp.send();                       
                            }
                            }  
function filter() {
    var keyword = document.getElementById("customer_fname").value;
    var select = document.getElementById("customer_id");
    for (var i = 0; i < select.length; i++) {
        var txt = select.options[i].text;
        if (txt.substring(0, keyword.length).toLowerCase() !== keyword.toLowerCase() && keyword.trim() !== "") {
            select.options[i].style.display = 'none';
        } else {
            select.options[i].style.display = 'list-item';
        }
    }
}
</script>    

<div class="form-group">
                                <label for="customer_id" class="col-sm-2 control-label">ชื่อลูกค้า</label>
                                <div class="col-sm-2">
                                    <input type="text" class="form-control" name="customer_fname"   id="customer_fname" placeholder="กรองชื่อลูกค้า"  onkeyup="filter()">
                                </div>       
                                <div class="col-sm-8">
                                    <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>
                                </div>
                            </div>