magento-add-comment-to-order.php

// get the last order
$lastOrderId = $this->_getOnepage()->getCheckout()->getLastOrderId();
$order = Mage::getModel('sales/order')->load($lastOrderId);
 
// Add the comment and save the order (last parameter will determine if comment will be sent to customer)
$order->addStatusHistoryComment('This comment is programatically added to last order in this Magento setup');
$order->save();