I wonder if there any better ideas to solve the problem below,
I have a form with a number of input fields, such as,
empty($var) is an abbreviation for !( isset($var) && $var ).
empty($var)
!( isset($var) && $var )
So !empty($_POST['...']) will be sufficient for your situation — the isset call you have currently is redundant.
!empty($_POST['...'])
isset