Magento: Session Success or Failure Message after redirect – Magento HowTo
Add a Success, Info or Error Message to the session and save this for re-direction in Magento
//A Success Message
Mage::getSingleton('checkout/session')->addSuccess("Your cart has been updated successfully!");
//A Error Message
Mage::getSingleton('checkout/session')->addError("Your cart has been updated successfully!");
//A Info Message (See link below)
Mage::getSingleton('checkout/session')->addNotice("This is just a FYI message...");
//These two lines are required to get it to work
session_write_close(); //THIS LINE IS VERY IMPORTANT!
$this->_redirect('checkout/cart');
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