I know this is old, but in case someone else lands here looking for a similar solution, this might be it:
The last three parameters in the execute array are indexed, when they need to have associative keys that match the prepared placeholders, like this:
$stmt->execute(array(
':FirstName' =>$_POST['FirstName'],
':password'=>$_POST['password'],
':email'=>$_POST['email'],
':duedate'=>$_POST['dueDate']
));