Get URL parameter and change the area to display

theme.php

<?php
if (isset($_GET['area'])) $area =  $_GET['area'];

if ($area = 'a') {
  $a = new Area('Main_A');
  $a->display($c);
}

if ($area = 'b') {
  $a = new Area($area);
  $a->display('Main_B');
}

if ($area = 'c') {
  $a = new Area($area);
  $a->display('Main_C');
}
?>