<?php

define('BASE_PATH', dirname(dirname(__FILE__)));
define('APP_FOLDER','simpleadmin');

require_once BASE_PATH.'/lib/MysqliDb.php';
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "gm";
// create connection object

$db =new MysqliDb($servername,$username,$password,$dbname);?>

Fatal error: Call to undefined method MysqliDb::real_escape_string() in C:xampphtdocsGrovecus_register.php on line 11

<?php
require_once './config/config.php';
$msg = "";
use PHPMailerPHPMailerPHPMailer;

if (isset($_POST['submit'])) {

    $f_name = $db->real_escape_string($_POST['f_name']);
^ in the line of code above is where the error comes

function get_name($email_add) {

   global $conn;

   $sql = "SELECT * FROM tbl_customers WHERE email_add = '$email_add'";

   $result = $conn->query($sql);

   $row = $result->fetch_assoc();

   return $row["id"];//try to use id instead of f_name

}

<?php
     include 'Config.php'; //The file with that code
     //Code with $conn variable
?>