How to define 'do_unset(...)' for a value in the 'userfield' table for 'add member'?

[timetunnel]

Hello.

To use this function (method) properly, how can I define '$this->{$this->table}' as the 'userfield' table in the following statement:

~~~~~~~~~~~
if (isset($this->{$this->table}["$fieldname"]))
~~~~~~~~~~~

when at the 'register_addmember_process' hook location in register.php?

During the 'add member' process, I want to 'UNSET' a value in the 'userfield' table via the hook, 'register_addmember_process' if, when using regex, it finds the input text field's default value is still in the field after the form is submitted.

I want to keep the userfield as required, but only under 1 specific condition.

Thanks in advance.