SELECT p.price, p.model,
( SELECT ps.price as sp_price
FROM product_special AS ps
WHERE NOW() BETWEEN ps.date_start AND ps.date_end
AND ps.product_id = "'.$product_id.'"
LIMIT 1
) as sp_price
FROM product as p
WHERE p.quantity > 0 AND p.product_id = "'.$product_id.'"<html>
<body>
<form>
<input id="hide2" name="admin" type="button" value="Администратор"
onclick="document.forms[0].user_type.value=this.name">
<input id="hide3" name="user" type="button" value="Пользователь"
onclick="document.forms[0].user_type.value=this.name">
<input type="text" name="user_type"><!-- для наглядности, заменить на type="hidden" -->
</form>
</body>
</html>