使用php从选择下拉列表中保存sql变量

i have the following select in my code. now it posts the whole value to the next page, but how can i pass the variable $dagen to the next page?

<form method="post" action="pagina4.php?lang=<?=$_SESSION['lang'];?>&naam=<?=$naam;?>&fietskeuze=<?=$fietskeuze;?>&opties=<?=$opties;?>&optieid=<?=$optieid;?>">
        <select id="dagen" name="dagen" style="width:250px;" style="height:250px;">
            <?php
            $query="SELECT * FROM $tabel WHERE fietstype='$fietskeuze'";
            $result=mysql_query($query) or die ("Ophalen prijzen mislukt: ".mysql_error());

            while ($row=mysql_fetch_array($result)) {
            $dagen=$row[dagen];
            $prijs=$row[prijs];
                echo "<option>
                    $dagen ".$lang['pagina3_dag']." - Tarief: EUR   ".$prijs."  
                </option>";
            }  

You already are. Some of your variables are available in $_GET (the query string of the action attribute), but the dagen variable will be available in $_POST['dagen'].

If the complete string is not what you want, you can add a value attribute to get just your $dagen variable:

 echo "<option value="<?php echo $dagen; ?>">
     ...