Stackoverflow | PHP Login Display Problem

<?php
session_start();

$user["usernamehere"] = "passwordhere";
if (!isset($_SESSION['logged_in']))
{
    if ($_SERVER['REQUEST_METHOD'] == 'POST')
    {
        if (empty($_POST['username']) || empty($_POST['password']))
        {
            echo '<html><body><div id="center"><h2> Login Status: <br/></h2><span style="color:red; font-weight: bold">Please fill in all fields!</span></div></body>';
header("Refresh: 0;");
        }
        elseif ($user[$_POST['username']] != $_POST['password'])
        {
            echo '<div id="center"><h2> Login Status: <br/></h2><span style="color:red; font-weight: bold">Your username/password is wrong!</span></div></body>';
header("Refresh: 0;");
        }
        else
        {
            $_SESSION['ingelogd'] = true;
            echo '<div id="center"><h2> Login Status: <br/></h2><span style="color:green; font-weight: bold">You are now logged in!</span></div>';
header("Location: /index.html");
        }
    }
    else
    {
        exit('<br /><br />
<link rel="stylesheet" href="/css/body.css">
<link rel="stylesheet" href="/css/button.css">
<div id="center">
        <form method="POST" action=""><p>
        Username:<br />
        <input type="text" name="username" /><br /><br />
        Password:<br />
        <input type="password" name="password" /><br /><br />
        <input type="submit" value="Login" /> <input type="reset" value="Empty fields" />
        </form></div>');
    }
}
?>
<link rel="stylesheet" href="/css/body.css">