<?php
include("connect.php");
session_start();
if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }
  if(isset($_SESSION["Username"]))
  {
    $user = $_SESSION["Username"];
    $pass = $_SESSION["Password"];
  }
  else
  {
    $user = $_POST["User"];
    $pass = $_POST["Password"];
    $_SESSION['Username'] = $user;
    $_SESSION['Password'] = $pass;
    $escuser = mysql_real_escape_string($user);
    $escpass = mysql_real_escape_string($pass);
  }
  $query = "SELECT * FROM ГДЕ_АККАУНТЫ WHERE ТУТ_ПЕРЕМЕННАЯ_НИКА_ВБД = '$escuser'";
  $result = mysql_query($query);
  $username_exist = mysql_num_rows($result);
  if($username_exist == 0)
  {
    echo 'Оо! <br />';
    echo '<a href="index.php">Выход</a>';
    unset($_SESSION['Username']);
    unset($_SESSION['Password']);
    die;
  }
  $row = mysql_fetch_row($result);
  if($row[2] !== $escpass)
  {
     echo 'Оо! <br />';
    echo '<a href="index.php">Выход</a>';
    unset($_SESSION['Username']);
    unset($_SESSION['Password']);
    die;
  }

$message = "Привет $escuser!<br />";
echo $message;
echo "<br />";
echo "
<table border = 1>
  <tr>
    <td>Тут переменная к примеру:</td>
    <td>$row['Параметр вбд']</td>
  </tr>
</table>";
?>