PHP Login Code

<?php
$group = "";
$rights = 0;
$regis = false;

if(isset($_GET["page"])){
    if($_GET["page"] == "login"){
        if(isset($_POST["username"]) and isset($_POST["password"])){
            $username = $_POST["username"];
            $password = $_POST["password"];
            $stmLogin = "SELECT * FROM acc WHERE password='".md5($password)."' AND username='$username';";
            $resLogin = $con->query($stmLogin);
            if($resLogin){
                if($resLogin->num_rows == 1){
                    $_SESSION["username"] = $username;
                    $_SESSION["password"] = $password;
                }else{
                    error("Password or username incorrect!");
                }
            }else{
                error("Error in stmLogin: $stmLogin");
            }
        }else{
            error("Login Error: couldn't send username or password");
        }
    }else if($_GET["page"] == "logout"){
        session_destroy();
    }else if($_GET["page"] == "regi"){
        if(isset($_POST["username"]) and isset($_POST["password"])){
            $username = $_POST["username"];
            $password = $_POST["password"];
            $stmNextId = "SELECT * FROM acc";
            $resNextId = $con->query($stmNextId);
            $id = $resNextId->num_rows;
            $stmRegi = "INSERT INTO acc VALUES($id,'$username','".md5($password)."', 2)";
            $resRegi = $con->query($stmRegi);
            if($resRegi === false){
                error("Couldn't register!");
                unset($_SESSION["username"]);
                unset($_SESSION["password"]);
            }else{
                error("Registered!");
                $_SESSION["username"] = $username;
                $_SESSION["password"] = $password;
            }

        }else{
            error("Login Error: couldn't send username or password");
        }
    }
    ?>
    <meta http-equiv="refresh" content="0; URL=<?php echo $filename ?>">
    <?php
}else if(isset($_SESSION["username"]) and isset($_SESSION["password"])){
    $username = $_SESSION["username"];
    $password = $_SESSION["password"];
    $stmLogin = "SELECT * FROM acc WHERE username='$username' AND password='".md5($password)."';";
    $resLogin = $con->query($stmLogin);
    if($resLogin){
        if($resLogin->num_rows == 1){
            $itemRights = $resLogin->fetch_assoc();
            $rights = $itemRechte["rights"];
            $regis = $rights==1;
            $group = $con->query("SELECT * FROM rechte_liste WHERE id=$rights")->fetch_assoc()["name"];

        }else{
            $rights = 0;
        }
    }
}
?>