HTML / PHP - 表单选择框大于数据库值，在每个选择值中增加500

I am trying to create something similar to an auction/bidding table/form. If a value (current bid) is 1000, I am trying to create a selection box that allows another user to bid in intervals of 500, so for this example the outcome would be like:

Current Bid: 1000
Buy Now: 5000
<select class="form-control">
<option value="1500">1500</option>
<option value="2000">2000</option>
<option value="2500">2500</option>
<option value="3000">3000</option>
<option value="3500">3500</option>
<option value="4000">4000</option>
<option value="4500">4500</option>
</select>

Something like this:

<?php
$query = $db->query('SELECT * FROM auctions WHERE available = 1 LIMIT 1');
$num = $query->num_rows;
if($num > 0) {
echo '<select class="form-control">';

foreach($query as $row) {

$currentBid = $row['currentBid']; // 1000
$buyNow = $row['buyNow']; // 5000


$bids = ?? // this is where I am stuck, how can I make the difference between $currentBid and $buyNow show as options divided by 500's


echo '
<option value="'.$bids.'">'.$bids.'</option>
';


}

echo '</select>';


}
else {
echo "No auctions available";
}

?>