<?php
   $id = '100'; // Replace id with your product id
   $qty = '2'; // Replace qty with your qty
   $_product = Mage::getModel('catalog/product')->load($id);
   $cart = Mage::getModel('checkout/cart');
   $cart->init();
   $cart->addProduct($_product, array('qty' => $qty));
   $cart->save();
   Mage::getSingleton('checkout/session')->setCartWasUpdated(true);
   ?>