profesor08

class Form1
{
  ...
  public void buttonClick(object sernder, EventArgs arg)
  {
    ...
    form2.UpdateFields(name, lastName);
    form2.Show();
  }
  ...
}

class Form2
{

  ...
  public void UpdateFields(string name, string lastname)
  {
     nameTextBox.Text  = name;
     lastNameTextBox.Text = lastname;
  }
  ...
}

error_reporting(E_ALL);
ini_set("display_errors", true);

$message = "
<span>Логин</span> {$LOGIN} <br>
<span>Пароль</span> {$PASS} <br>
";

ob_start();
include MAIL.'registrationSuccess.php';
$message = ob_get_contents();
ob_end_clean();