Login (zřejmě se nespustí session)

<?php
if (login() == true) {
    ?>
    <a href="index.php">Home</a><br>
    <a href="profile.php">Profile</a><br>
    <a href="logout.php">Log Out</a>
    <?php
} else {
    ?>
    <a href="index.php">Home</a><br>
    <a href="login.php">Login</a><br>
    <a href="register.php">Register</a>
    <?php
}
?>

<?php

session_start();

function login(){
    if (isset($_SESSION["user_id"]) && !empty($_SESSION["user_id"]) ) {
        return true;
    } else {
        return false;
    }
}

?>

<form method="post">
                <?php
                if (isset($_POST["submit"])) {
                    $username = $_POST["username"];
                    $password = md5($_POST["password"]);

                    if (empty("$username") || empty("$password")) {
                        echo "<p>Vyplňte všechna pole!</p>";
                    } else {
                        $check_login = mysql_query("SELECT id FROM users WHERE username = '$username' AND password = '$password'");
                        if (mysql_num_rows($check_login) == 1) {
                            $run = mysql_fetch_array($check_login);
                            $user_id = $run["id"];
                            $user_type = $run["type"];
                            if ($user_type == "d") {
                                echo 'Tvůj účet je deaktivován';
                            } else {
                                $_SESSION_ = ["$user_id"];
                                header("location: index.php");
                            }
                        }
                    }
                }
                ?>
                Uživatelské jméno:<br>
                <input type="text" name="username"/><br>
                Heslo:<br>
                <input type="password" name="password"/><br><br>
                <input type="submit" name="submit" value="Login"/>
            </form>