Magento – How to Get Order Details by Invoice ID

Given $order, I can get the email from the order like this:

$customer_email = $order->getCustomerEmail();

Then I can get the account details with the following code:

$customer = Mage::getModel("customer/customer"); 
$customer->setWebsiteId(Mage::app()->getWebsite()->getId()); 
$customer->loadByEmail($customer_email); 
echo $customer->getId(); 
echo $customer->getFirstName(); 
print_r($customer->getData());  

Credit: http://www.techdilate.com/code/magento-get-customer-details-by-email-id/

James As you want you works in on invoice creation time you can use getOrigData('entity_id')

On creation on invoice it always give the null value and you can use sales_order_invoice_save_after events

so you try use

$invoiceId = $observer->getEvent()->getInvoice()->getOrigData('entity_id');
if(is_null($invoiceId )){
// do your logic
}

an for getting item qty try this

   foreach ($invoice->getAllItems() as $item){
                if ($item->getOrderItem()->getParentItem()) {
                    continue;
                }
                /* Draw item */

        $name = $item->getName();
        $type = $item->getSku();
        $id = $item->getProductId();
        $qty = $item->getQty();

        }