How can I get custom user registration field parameters
From the onSaveUser event docs, there are two parameters that are passed to everyone listening.  One is isNewUser, which you're accessing correctly: $event-&gt;params['isNewUser'].  But the other is user, which is a full-blown UserModel object that you can access the properties of.

But onSaveUser isn't the event you want, since you want to make changes to the user before they've been saved.  You want onBeforeSaveUser, which has the exact same parameters.

Having said that, names are tricky things around the globe.  I highly recommend reading this article on falsehoods that programmers believe about names.

Here is some code that should work for the most basic use cases.

craft()-&gt;on('users.saveUser', function (Event $event) {

    // Only do this for new users.
    if ($event-&gt;params['isNewUser'])
    {
        $fullName = craft()-&gt;request-&gt;getPost('fields.fullName');

        // Make sure this isn't a user being saved from the control panel.
        if ($fullName)
        {
            $name = explode(' ', $fullName);

            // No spaces, it could be Madonna or Cher.
            if (count($name) == 1)
            {
                $firstName = $name[0];
                $lastName = '';
            }
            // It's a "normal" first and last name.
            else if ($count($name) == 2)
            {
                $firstName = $name[0];
                $lastName = $name[1];
            }
            else
            {
                // The first element is the first name. Combine the rest into a last name separated by spaces.
                $firstName = array_pop($name);
                $lastName = implode(' ', $name);
            }

            // Grab the user.
            $user = $event-&gt;params['user'];

            $user-&gt;firstName = $firstName;
            $user-&gt;lastName = $lastName;
        }
    }
}


Note that UserModel already has a getFullName method that does some basic logic combining the firstName and lastName properties.