inhoud verdwijnt als optie niet is geselecteerd

<?php
$datum = date("d.m.y");
$tijd = strftime("%H:%M:%S");
$bericht = htmlentities($_POST[bericht]);
$bericht = nl2br($bericht);
$bericht = stripslashes($bericht);


$file = "gastenboek.txt";
$fopenr = fopen($file, "r");
$jouwtekst = fread($fopenr, filesize($file));
$fopenw = fopen($file, "w+");


$file2 = "Abonnees.txt";
$fopenr2 = fopen($file2, "r");
$jouwtekst2 = fread($fopenr2, filesize($file2));
$fopenw2 = fopen($file2, "w+");


if ($_SERVER[REMOTE_ADDR] == '::1') {
    $ip = "127.0.0.1";
} else {
    $ip = $_SERVER[REMOTE_ADDR];
}


fwrite($fopenw,"
<fieldset class='show'>
	<div class='show'>
		<table border='0' class='show' width='100%'>
		<tr>
			<td align='center'>
				<strong>Geplaatst door $_POST[naam]<i>(IP: $ip)</i> op $datum om $tijd</strong>
			</td>
		</tr>
		</table>
	</div>
	<div class='text'>
		$bericht
	</div>
</fieldset>
$jouwtekst");
fclose($fopenw);


if ($_POST['abonneren'] == "ja") {
    if ($_POST['email'] == "") {
        fwrite($fopenw2, "$_POST[naam](geen email)
$jouwtekst2");
        fclose($fopenw2);
    } else {
        fwrite($fopenw2, "$_POST[naam]($_POST[email])
$jouwtekst2");
        fclose($fopenw2);
    }
}


header('Location: gastenboek.php');
?>

<h2>Gastenboek</h2>


        <div id="tekstblok">
            <table width='1000'>
                <tr>
                    <td>
                        <form action="verwerking.php" method="post">
                            <center><font size="6">Bericht plaatsen:</font>
                                <br>
                                <div class="grid nopadding group commenter-inputs">
                                    <div class="grid-1">
                                        <input placeholder="Naam (vereist)" required="" type="text" name="naam" value="" class="infoput" maxlength="25">
                                    </div>
                                    <div class="grid-1">
                                        <input placeholder="Email (optioneel)" type="email" name="email" value="" class="infoput" maxlength="25">
                                    </div>
                                </div>
                                <textarea required="" name="bericht" id="textarea" cols="40" rows="3" placeholder="Uw bericht hier" maxlength="240"></textarea>
                                <p STYLE="margin-bottom: 7; margin-top: 5;"><strong>Abonneren?</strong> &nbsp;&nbsp;
                                    <input type="radio" name="abonneren" value="ja">Ja, graag! &nbsp;&nbsp;
                                    <input type="radio" name="abonneren" value="nee" checked>Nee bedankt
                                    <br />
                                </p>
                                <input class="button" type="submit" name="submit" value="Verstuur" style="font-size:125%; width:30%">&nbsp;&nbsp;&nbsp;
                                <input class="button" type="reset" value="Opnieuw" style="font-size:125%; width:30%">
                            </center>
                    </td>
                    <td style="vertical-align: top;">
                        <center><font size="6">Abonnees:</font>
                            <br>
                            <textarea name="abonnees" id="textarea" cols="35" rows="9" readonly="readonly"><?php
                              $file = fopen("Abonnees.txt","r")or exit("Kan bestand niet openen!");
                            while(!feof($file))
                              {
                              echo fgets($file);
                              }
                            fclose($file);
                            ?></textarea>
                        </center>
                    </td>
                </tr>
            </table>
            <div style="height:0;font:0/0 serif;border-bottom:3px dashed blue"></div>
            </form>
    </body>


</html>