Form info won't transfer into if (isset) conditional

Trying to write form info to file. If(isset) writes strings to file but will not write form info to file. It cannot recognise sessions request or Post variables. what can I do? In this code I tried to write the variable $info to file but nothing happens. It writes to screen outside if (isset) just fine.

Code:

<?php
        session_start();

        include 'Klasser.php';
        $Pers = new Kunde();
        $Best = new Bestilling();
        $Persinfo = $Pers->tilStreng();
        $Filminfo = $Best->tilStreng();
        $info = $Persinfo . "</br>" . $Filminfo;
        $send = $_REQUEST["email"];
        echo "Følgende valg er gjort :</br>";
        echo $info;

        $_SESSION["name_id"] =$_REQUEST["name_id"];
        $_SESSION["email"] =$_REQUEST["email"];
        $_SESSION["dato"] =$_REQUEST["dato"];
        $_SESSION["show"] =$_REQUEST["show"];

        ?>

        </br>

        <form action="" method="POST">
            Stemmer bestillingen? 
            <input type="checkbox" value='' name='box' />
            <input type="submit" value="Bekreft bestilling" name="knapp" />
        </form>
        <a href="index.php">Forandre på bestilling</a>

        <?php 
        if (isset($_POST["knapp"])) {
            mail($send, "Bekreftelse av Kinobestilling", $info);

            echo "<br/>Brekreftelse sendt";
            $filref = fopen("Bestilt.txt", "w");
            fwrite($filref, "Ny Bestilling<br/>".$info);
            fclose($filref);
        }
     ?>