PHP Blocking access to the login page

I just want to Limit the number of login attempts
(after three attempts the user will be locked out)

I have the users and passwords in the database already

<?php
// *** Validate request to login to this site.
if (!isset($_SESSION)) {
  session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
  $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['userName'])) {
  $loginUsername=$_POST['userName'];
  $password=$_POST['password'];
  $MM_fldUserAuthorization = "";
  $MM_redirectLoginSuccess = "index.php";
  $MM_redirectLoginFailed = "login.php";
  $MM_redirecttoReferrer = false;
  mysql_select_db($database_conn_helpdesk, $conn_helpdesk);

  $LoginRS__query=sprintf("SELECT userName, password FROM technician WHERE userName=%s AND password=%s",
    GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text")); 
  $LoginRS = mysql_query($LoginRS__query, $conn_helpdesk) or die(mysql_error());

  $loginFoundUser = mysql_num_rows($LoginRS);
  if ($loginFoundUser) {
     $loginStrGroup = "";

    if (PHP_VERSION >= 5.1) {session_regenerate_id(true);} else {session_regenerate_id();}
    //declare two session variables and assign them
    $_SESSION['MM_Username'] = $loginUsername;
    $_SESSION['MM_UserGroup'] = $loginStrGroup;       

    if (isset($_SESSION['PrevUrl']) && false) {

      $MM_redirectLoginSuccess = $_SESSION['PrevUrl'];  
      echo "erorr";
    }
    header("Location: " . $MM_redirectLoginSuccess );
  }
}
?>

How can implement that into my code?
PS: This code was generated by Dreamweaver.