<form action="sell.php" method="post">
    <fieldset>
        <div class="form-group">
            Stock:
            <select name = "stock">
            <?php foreach($stocks as $stock):?>
                    print("<option value=\"{$stock}\">{$stock}</option>");
            <?php endforeach ?>
            </select>
        </div>
    </fieldset>
</form>