Logg inn shit

<?php
session_start();

if(isset($_POST)&&!empty($_POST) {
    $_username = "knut";
    $_password = "123";

    $username = $_POST['username'];
    $password = $_POST['password'];

    if($username == $_username && $password == $_password) {
        $_SESSION["loggedin"] = "1";
    }
}
if(!isset($_SESSION["loggedin"])) {
    <form action="">
        <input type="text" placeholder="Brukarnavn"></input>
        <input type="password" placeholder="Passord"></input>
        <input type="submit" value="Logg inn">
    </form>
} else {
    <form action="LINK TIL SCRIPT">
        <input type="submit" value="Koyr script">
    </form>
}
?>