This question $_POST is not working in ajax form submit? had six answers at the time the bounty was placed. The question has been answered in these answers.
This is the original question:
When I try to check user input name is already exist by ajax form submit !But it only get Undefined index: username in sessions.php ,what is missing ?
This it the added question in the bounty:
Looking for an answer drawing from credible and/or official sources.
explanation about contentType and processData usage in ajax
To me this is a similar issue to a question being edited to invalidate the current answers and this How much change to the question is too much? gives instruction on how to handle that, except the question is being changed by using a bounty.
I have mod flagged the question:
Is this an acceptable use of the bounty feature?
Is a mod flag acceptable?
If not, what is the best course of action to take?
These questions are related but don't give an instruction on how best to handle this:
Can a bounty ask to do things not directly related to the original question?
How to ask for a different answer to an existing question? A bounty?