if (isset ($_POST['genresorteer'])){
    if($_POST['genresorteer'] > 0){
        $query = mysql_query("SELECT serienaam FROM serie WHERE genrenummer =    ".$_POST['genrenummer']);
    } else {
        $query = mysql_query("SELECT * FROM serie ORDER BY serienaam ASC ");
    }
} else {
    $query = mysql_query("SELECT * FROM serie ORDER BY serienaam ASC ");
}
while ($result = mysql_fetch_array($query)){
    echo "<a href='serie.php?serienummer=".$result['serienummer']."'>".$result["serienaam"]."</a>"."<br>";
}