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localhost/login could trigger a Login controller's index(), or it could be a login() method inside Configure, or it could even be the default app inside a "login" folder (thought that sounds awkward if "Configure" is your default).

If the intent of the login function is to return JSON, then I would set it up as a separate controller, though that is not a requirement.

If a method inside Configure, its signature would be login($email).
If a stand-alone controller, its signature would be Login::index() and you would need to pick off the email parameter explicitly.
If a stand-along controller, but you use a POST, then the signature would be Login::index() and you would use $this->input->post('email') to retrieve the parameter.

In all these cases, the logic would be something like:

Code:

 ... validate/retrieve user info
 header("Content-type: application/json");
 echo json_encode($user_info_to_return);

If "subit_backend" is referring to a controller, then "localhost/subit_backend" would normally resolve to "application/controllers/Subit_backend.php"

Hope this helps.