Upgrade to Magento 1.9.x and find your custom registration forms not working?

We’ve come across this a couple of times recently wherein an upgrade to Magento results in registration forms not working as they should be. The symptoms this time around were as follows:

1. Visitor fills out registration form and clicks submit
2. Visitor is brought back to the registration form as if nothing has happened
3. And nothing has happened — the registration details were never actually successfully sent to the server

Unfortunately, you’ll find no logging in the server error logs about this. I couldn’t find magento logs indicating a problem either.

The solution, as found on the Magento Forums, is to find your registration form template file. In my case it was here (the clarion folder because this particular site is using the Clarion Customer Attributes plugin):

{webroot}/app/design/frontend/{template_name}/default/template/clarion/customerattribute/form/register.phtml

Edit the file, and directly underneath the opening <form […]> tag, enter:

<input type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey() ?>" />

You may need to clear your cache in the Magento admin, though in my case it took effect immediately. After making that change I was able to successfully submit the registration form.