put the contents of one field into another field upon form submit

Question

how can I add a field to a form submission upon submit that will have the same value as the field name "mail" but with the name "email"

Answer 1

I think it'll work if you implement hook_form_submit() hook for this form, and in that function set the value of one field to another.

E.g.,

FORM_ID_form_submit($form, &$form_state) {
  $form_state['values']['email'] = $form_state['values']['mail']; 
}

If your fields are fields of a content then replace the above line with this:

$form_state['values']['email']['und'][0]['value'] = $form_state['values']['mail']['und'][0]['value'];

Answer 2

In Drupal 8 I did it this way, there may be a shorter way though

$value=$form_state->getValue('field_name');
$number=($value[0]['value']);
$form_state->setValue('other_field',$number);