How to show error message on previous page?

I want to show an error on the same as page as my form. My form is going to another page and then processing and returning the result. How can I show the error on the previous page?

if($_POST['btnManage']=="Signup"){
    $objCustomer->Email=$_POST['txtEmail'];
    $objCustomer->Password=$_POST['txtPassword'];
    $Status=$objCustomer->Signup();
    if($Status>0)
    {
        session_start();
        $_SESSION['Email']=$objCustomer->Email;
        header("Location:../index.php");
    } else
    {
        echo "error";
    }
}