Password Protected Redirect Directories

Would there be a way to modify this so I could use it with multiple usernames and passwords so I could redirect a user to a specified directory that you could only see if you had logged in on a master page first.


<?php

// Define your username and password
$username = "someuser";
$password = "somepassword";

if ($_POST['txtUsername'] != $username || $_POST['txtPassword'] != $password) {

?>

<h1>Login</h1>

<form name="form" method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <p><label for="txtUsername">Username:</label>
    <br /><input type="text" title="Enter your Username" name="txtUsername" /></p>

    <p><label for="txtpassword">Password:</label>
    <br /><input type="password" title="Enter your password" name="txtPassword" /></p>

    <p><input type="submit" name="Submit" value="Login" /></p>

</form>

<?php

}
else {

?>

<p>This is the protected page. Your private content goes here.</p>

<?php

}

?> 

If I change the bottom to


<?php

}
else {

header( 'Location: http://www.test.com/test' );

}

?> 

I can redirect the user but how can I do this for multiple users and how can I make sure they have to be logged in so they can’t just go to the URL in the first place?