<?php
session_start();

if(isset($_SESSION['username'])){
	echo "Logged in as: " . $_SESSION['username'];
}
else{
	echo '<form action="login.php" method="post">
		  Username: <input name="username" type="text" /><br>
		  Password: <input type="password" name="password" /><br>
		  <button type="submit">Submit</button><br>
		  </form>';
}

mysql_connect("HOST", "USER", "PASS");
mysql_select_db("DATABASE");

if(isset($_POST['username'])){

	if(login($_POST['username'], $_POST['password'])){
		echo "Logged in as: ".$_POST['username'];
		$_SESSION['username'] = $_POST['username'];
	}

}

      function login($username, $password){
              $username = mysql_real_escape_string($username);
              $password = md5($password);

              $result = mysql_query("SELECT *  FROM `users` WHERE `username` = '$username' AND `pass` LIKE '$password'");

              $row = mysql_fetch_row($result);

              if($row[2] == $password){
                      return true;
              }
              else{
                      return false;
              }
      }
?>