Magento - apply coupon codes automatically

product.phtml

$productPrice = $_product->getPrice();
$coupon_code = trim(Mage::getSingleton("checkout/session")->getData("coupon_code"));

if ($coupon_code != '') {
	/*** Warning - if you choose to use this code, you should probably not
	 *  use the direct SQL query below, and instead use the SalesRule/Rule
	 *  resource collection, which you can access by calling
	 *  Mage::getModel('salesrule/rule')->getResourceCollection()
	 *  
	 *  This example was written a long time ago and the direct SQL query was
	 *  the best I could do given my limited understanding of Magento at the time.
	***/
	$db = Mage::getSingleton('core/resource')->getConnection('core_read');
	$sql = "SELECT discount_amount, conditions_serialized
		FROM salesrule_coupon AS a INNER JOIN salesrule AS b ON a.rule_id = b.rule_id
		WHERE simple_action = 'by_percent' AND a.code = ?";
	$sql = $db->quoteInto($sql, $coupon_code);
	$rows = $db->fetchAll($sql);
	if (isset($rows[0])) {
		$discount = ($rows[0]['discount_amount'] / 100);
		
		/*** This assumes that if there is a shopping cart price rule condition,
		  *  it is a condition with only one parameter - that the product belongs
		  *  to a particular attribute set.
		***/
		$conditions = unserialize($rows[0]['conditions_serialized']);
		$conditions = $conditions['conditions'][0]['conditions'][0];
		if (is_array($conditions) && $conditions['value'] == $_product->getAttributeSetId()) {
			$productPrice = round($productPrice - ($productPrice * $discount),2);
			$sale_price = true;
		}
	}
}