调用未定义的方法MyDbCon

Question

好吧我收到错误调用未定义的方法MyDbCon而我是新手所以请帮助我，我在这里做错了什么？温柔哈哈

use Zend\Db\Sql\Select;
function getStudentsByMst(&$response,$mst_id,$stud_id=true,$now=true)
{   if(!ctype_digit($mst_id))
    {   $response = array(
                        'code' => HTTP_Status::BAD_REQUEST,
                        'message' => 'Registration ID cannot be digits only'
                        );
        return false;
    }
}
include_once('credentials.db.php'); 
if(empty($_POST['stud_id']) == FALSE)
{
$barcode    =   htmlentities($_POST['stud_id']);
//checking if the barcode exist in the database 
    $dbh=new MyDbCon;
    $barcode_check  = $dbh->query("select * from usats where stud_id='$stud_enrolmentno'");
    //extracting the status of the item
    if($barcode_check->rowCount() > 0)
    {
        while($row = $barcode_check->fetch(PDO::FETCH_OBJ))
        {
            $presence = $row->presence;
        }
    }
    if($barcode_check->rowCount() > 0 && $presence == '1')
    {
        //updating the database table barcode for student presence/absence
        $addAttd    =   $dbh->exec("UPDATE usats SET status='0' where stud_id= '$stud_enrolmentno'");
        if($addAttd > 0)
        {
            echo "<script> alert('Successful');</script>";
        }
    }
}

?>