Cannot echo "username" from SESSION

[hilda]

Hi here is the login page

<?
session_start();  
session_register("login_session");  
$username = "";
$password = "";

if (isset($_POST["Username"]))
   $username = $_POST["Username"];
if (isset($_POST["Password"]))
   $password = $_POST["Password"];

if ($username != "" && $password != "") {

   $link = mysql_connect("localhost","root","root");
   mysql_select_db("database"); 

   $sql = "SELECT * FROM trainee WHERE password='";
   $sql.= $password."' AND username='".$username."'";
   $result = mysql_query($sql); 
   if (mysql_fetch_row($result) != false) {

      $_SESSION["login_session"] = true;
      if ($username = "trainee"){
      header("Location: student.php");}
    }else 
        echo '<script type="text/javascript">
        window.onload=function(){alert("It is incorrect! Please enter again!");} 
        </script>';
   mysql_close($link);
}
?>

Below is the form:

<div class="sidebar">
          <div class="sidebar_item" id="login_form">
            <p>&nbsp;</p>
            <form method="post" action="login_stu.php">
			<fieldset>
			<legend align="center">Trainee Login</legend>
              <p>
                <label for="ID"> Username:</label>
                <input type="text" name="Username" />
              </p>
              <p>
                <label for="PASSWORD"> Password:<br />
                </label>
                <input type="password" name="Password" />
              </p>
				<p align="center">
                <input type="submit" name="submit" value="Login" style="height:1.7em; width:3.5em;"/>
				</p>
			  </fieldset>
            </form>
          </div> 
        </div> 

I got another page requiring me to display the username
And I have tried to use

session_start(); 
echo $_SESSION['username'] 

or

session_start(); 
echo $username; 

, etc.

Any ideas? Thank you.

[simplypixie]

You aren't assigning the username to a session variable anywhere, so you need to do so

if (mysql_fetch_row($result) != false) { 

      $_SESSION["login_session"] = true;
      $_SESSION['username'] = $username;