function testCalculateCharge()
 {
     $item = new Item('Perfect PHP', 3600, '2010/11', 2);
     $this->assertIdentical($item->calculateCharge(1), 3600);
     $this->assertIdentical($item->calculateCharge(2), 7200);
     $item = new Item('Perfect PHP', 500, '2010/11', 2);
     $this->assertIdentical($item->calculateCharge(1), 500);
     $this->assertIdentical($item->calculateCharge(2), 1000);
     $this->assertFalse($item->calculateCharge(0));
     $this->assertFalse($item->calculateCharge('string'));
     $this->assertFalse($item->calculateCharge(1.1));
     $this->assertFalse($item->calculateCharge(null));
 }