if(isset($_POST['prof_match']) && ($_POST['prof_match']) != 'NULL') {
  $choice_prof = "= " . ($_POST['prof_match']); // Example P01 is Accountant
}else {
  $choice_prof = 'IN(SELECT prof FROM profiles)';
}

// The query is as follows:

SELECT * FROM profiles WHERE age $choice_age AND height $choice_ht AND edn $choice_edn AND prof $choice_prof