put this on the TOP of your login.php:
if (($username=='username')&&($password=='password')){
Header("location:nextpage.php");
}else{
Header("location:login.php");
Then the form in your body:
<form method=post action=login.php>
<input type=text name=username>
<input type=password name=password>
<input type=submit>
</form>
haven't tested...
you can only redirect to a page using php's Header function, cannot do ASP's redirect