How can I add the customer login & registration form data in my log?

I want to add the form data in log files when the customer submits the login/registration form on my Magento 1 store. How can I do it using login.phmtl and register.phtml template files?

I am using Magento ver. 1.9.4.4

I tried using the below method but not working.

<?php
    Mage::log(print_r($formData, 1), null, 'logfile.log',true);
?>

How do I fetch the form data in $formData variable?