<form class="" action="<?php $_SERVER['PHP_SELF'] ;?>" method="post">
<div class="input-group">
<select class="form-control input-lg" name="date1" id="date1" >
  <?php foreach($orders as $order ): ?>
    <option value="<?php echo $order['id']; ?>"><?php echo $order['order_date']; ?></option>
  <?php endforeach; ?>
</select>
<select class="form-control input-lg" name="date2" >
  <?php foreach($orders as $order): ?>
    <option value="<?php echo $order['id']; ?>"><?php echo $order['order_date']; ?></option>
  <?php endforeach; ?>
</select>
</div>
 <input type="submit" value="Get Total">
 <input type="hidden" value="" name="date_text1" id="date_text1">
 <input type="hidden" value="" name="date_text2" id="date_text2">
</form>