<form action='' method='post'>
 Vardas: <input type='text' name='user_name'> <br />
 Taškai: <input type='text' name='user_taskai'> <br />
 <input type='submit' value='prideti' name='prideti'>
 </form>
 
 <?php
 if (isset($_POST['prideti'])) {
 dbquery("UPDATE ".$db_prefix."users SET users_taskai = users_taskai - ".$_POST['user_taskai']." WHERE user_name='".$_POST['user_name']."' ");
 }
 ?>