It works on input fields, but not working in select fields.

So far this code is working only in input field, my select fields are: region,province and city the rest are inputs

<script>
    function GetDetail(str) { 
        if (str.length == 0) { 
            document.getElementById("firstname").value = ""; 
            document.getElementById("lastname").value = ""; 
            document.getElementById("email").value = ""; 
            document.getElementById("region").value = ""; 
            document.getElementById("province").value = ""; 
            document.getElementById("city").value = ""; 
            document.getElementById("storename").value = ""; 
            return; 
        } 
        else { 

            // Creates a new XMLHttpRequest object 
            var xmlhttp = new XMLHttpRequest(); 
            xmlhttp.onreadystatechange = function () { 

            if (this.readyState == 4 && 
                this.status == 200) { 
        
                var myObj = JSON.parse(this.responseText); 
        
                document.getElementById("firstname").value = myObj[0]; 
                document.getElementById("lastname").value = myObj[1]; 
                document.getElementById("email").value = myObj[2]; 
                document.getElementById("region").value = myObj[3]; 
                document.getElementById("province").value = myObj[4]; 
                document.getElementById("city").value = myObj[5]; 
                document.getElementById("storename").value = myObj[6]; 
           }         
       }; 

          // xhttp.open("GET", "filename", true); 
          xmlhttp.open("GET", "gfg.php?mobile=" + str, true); 
    
          // Sends the request to the server 
          xmlhttp.send(); 
      } 
    } 

</script>

<?php 

include 'server/config.php';
// Get the user id 
$user_id = $_REQUEST['mobile']; 


if ($user_id !== "") { 

$stmt = $database->prepare("SELECT firstname,lastname,email,region,province,city,storename FROM      records_db WHERE mobile = :mobile");
$stmt->bindParam(':mobile',$user_id);
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_OBJ);

$firstname  = $row->firstname;
$lastname   = $row->lastname;
$email      = $row->email;
$region     = $row->region;
$province   = $row->province;
$city       = $row->city;
$storename  = $row->storename;
} 

    // Store it in a array 
    $result = array("$firstname", "$lastname", "$email", "$region", "$province", "$city", "$storename"); 

    // Send in JSON encoded form 
    $myJSON = json_encode($result); 
    echo $myJSON; 
?>

Once I enter a mobile number, the records of that mobile number will automatically display on the input fields also in the select fields, but unfortunately the select fields are not working.