POST一个隐藏的输入+多个选项PHP

大家好，我是PHP新手

我有这个问题:

我想POST一个多选题+一个隐藏的字段从一个表单:

<?php
if (isset($_SESSION['nickname']))
{
    $result = mysql_query("SELECT * FROM users");
    $teamsCount = ceil(mysql_num_rows($result)/2);
    for ($i=1; $i<=$teamsCount; $i++)
    {
        // TEST: echo $i . " TeamsCount er: " . $teamsCount. "<br>"; 
?>
Team <? echo $i; ?> 
<form name="addTeam" action="buildTeams.php" method="POST">
    <input type="hidden" name="hiddenField" value="<?php $i; ?>" />
    <select name="teams[]" multiple="multiple" size="<?php echo mysql_num_rows($result); ?>">
    <?php 
        $query = mysql_query("SELECT * FROM users");
        while ($row=mysql_fetch_array($query)) 
        {
            $id=$row["ID"];
            $nick=$row["Nick"];
    ?>
    <option value="<?php echo $id; ?>"><?php echo ucfirst($nick); ?></option>
<?php   
        } 
?>
    </select>
    <input type="submit" value="Make them teams!!" />
</form>
<?php 
    }
} 
?>