$user='username';//name
$pass='pass';//password
$db='data';//database name
$pid='1';//product id
$database='product';//database name
$newstock='2';//new stock
$productid='1';//product id
$tablename='productdetail';//table name
//connect to database
$con=mysql_connect('localhost',$user,$pass);
//select database
mysql_select_db($database,$con);
//update stock query
$qur="update '$tablename' set stock='$newstock' where productid='$pid'";
//execute query
mysql_query($qur,$con);