Simple PHP Multi User login, different pages depending on user - what's wrong here?
<?php
session_start();
$data=array("user1"=>array("url"=>"file1.php","password"=>"pass1"),
"user2"=>array("url"=>"file2.php","password"=>"pass2"));

if(isset($_POST['username']) && isset($_POST['password'])) {
    if($data[$_POST['username']]['password'] == $_POST['password']) {
        $_SESSION['username'] = $_POST['username'] . " " . $_POST['password'];
        header('Location: ' . $data[$_POST['username']]['url']);

        login('<p class="alert">Incorrect username or password.</p> ');
    }
} else {
    login();
}
?><?php
function login($response='Please enter your username and password.') {
?>

<p><?=$response?></p>
<form action="" method="post">
    <table width="400" border="0" cellspacing="0" cellpadding="4">
        <tr>
            <td width="90"><label class="loginform" for="username">Username:</label></td>
            <td width="294"><input name="username" type="text" /></td>
        </tr>
        <tr>
            <td><label for="password">Password:</label></td>
            <td><input name="password" type="password"></td>
        </tr>
        <tr>
            <td>&nbsp;</td>
            <td><input type="submit" value="Login" /></td>
        </tr>
    </table>
</form>
<?php } ?>

<?php } ?>

if(isset($_POST['username']) && isset($_POST['password']))
{
    if($data[$_POST['username']]['password'] == $_POST['password'])
    {
        $_SESSION['username'] = $_POST['username'] . " " . $_POST['password'];
        header('Location: ' . $data[$_POST['username']]['url']);

        login('<p class="alert">Incorrect username or password.</p> ');
    }
} else {
    login();
}

if(isset($_POST['username']) && isset($_POST['password']))
{
    if($data[$_POST['username']]['password'] == $_POST['password'])
    {
        $_SESSION['username'] = $_POST['username'] . " " . $_POST['password'];
        header('Location: ' . $data[$_POST['username']]['url']);
        exit();
    }
    else
    {
        login('<p class="alert">Incorrect username or password.</p> ');
    }
} else {
    login();
}

$warning = '';

if(isset($_POST['username']) && isset($_POST['password']))
{
    if($data[$_POST['username']]['password'] == $_POST['password'])
    {
        $_SESSION['username'] = $_POST['username'] . " " . $_POST['password'];
        header('Location: ' . $data[$_POST['username']]['url']);
        exit();
    }
    else
    {
        $warning = '<p class="alert">Incorrect username or password.</p> ';
    }
}

login($warning);

if(isset($_POST['username']) && isset($_POST['password'])) {
if($data[$_POST['username']]['password'] == $_POST['password']) {
$_SESSION['username'] = $_POST['username'] . " " . $_POST['password'];
header('Location: ' . $data[$_POST['username']]['url']);

login('<p class="alert">Incorrect username or password.</p> ');

}
} else {
login();
}

if(isset($_POST['username']) && isset($_POST['password'])) {
if($data[$_POST['username']]['password'] == $_POST['password']) {
$_SESSION['username'] = $_POST['username'] . " " . $_POST['password'];
header('Location: ' . $data[$_POST['username']]['url']);

} else {
login('<p class="alert">Incorrect username or password.</p> ');
}
} else {
login();
}