Multi usernames during session

[Kudose]

Here is my issue. When I log in it gives me username 1. when I click back and log in with a different name it gives me welcome username1.

How can I get this script to recognize the different sessions to accept multiple users without logging out. When I click log out, which is just session_destroy(); it gives me username2 when I log in with a different username than the first time.

Thanks.

<?
session_start();
session_destroy();
include('dbinfo.inc.php');

$query = "SELECT * FROM users WHERE username='$username' AND password='$password'";
$result = mysql_query($query) or die(mysql_error());
$rows = mysql_num_rows($result) or die(mysql_error());

if ($rows == '1') {

session_start();
session_register("username");
header("Location: secure.php");

} else {

echo" <font color='#000000'>ACCESS DENIED! </font> ";

}


?>

Tjat was the verify.php

Here is the secure.php page:

<? 
session_start();
if (session_is_registered("username")) {

include('dbinfo.inc.php');
include('css.css');
include('top.php');


?>

    <tr> 
      <td width=593 height=1 colspan=2><img src="/images/black.gif" width=100% height=1></td>
    </tr>
    <tr> 
      <td width=593 height=1 colspan=2><img src="/images/black.gif" width=100% height=1></td>
    </tr>
  </table>

  <table cellpadding="0" cellspacing="0" border="0" width="656" align="center">


		<tr>
			<td colspan="2" width="621" bgcolor="#687B8C" valign="top">

			<table cellpadding="10" cellspacing="0" border="0" width="654">
    <tr> 
      <td valign="top"> <div align="center"></div>
        <table width="100%" border="0" align="center" cellpadding="0" cellspacing="0">
          <tr> 
            <td bgcolor="#415369" width="465"><img src="/images/leftcorners_smallboxes.gif" alt="" border="0" width="32" height="32"> 
              <font face="Tahoma" color="#ffffff" size="4"><b>Login Accepted!</b></font></td>
          </tr>
          <tr> 
            <td bgcolor="#415369" height="120" width="465"><div align="center"> 
                <font color="#FFFFFF" size="2" face="Tahoma"><strong>Welcome 
                <?= $username ?>
                </strong> </font></div></td>
          </tr>
        </table>
        &nbsp; </td>
    </tr>
  </table>

			</td>
		</tr>

	</table>
	</td>
</tr>
</table>
<?
include('bottom.php');
} else {
header("Location: [url]http://kaboom.ionichost.com/index.php[/url]");
}
?>

Thanks so much in advance.

[dalecosp]

Set your session vars this way:

$_SESSION['username']=$username;

Destroy them thusly:

unset ($_SESSION['username']);

Don't call session_start() twice.
Unset $SESSION['username'] before you set it:

if ($rows==1) {
    if ($_SESSION['username']) {
        unset $_SESSION['username'];
        }
    $_SESSION['username']=$username;
    header("Location: secure.php");
    }