PHPunit test: Second test onwards fails on db connection

I am writing my first phpunit test, here is class for test

require_once "../../configure.php";

class OrderTest extends PHPUnit_Framework_TestCase
{


    public function testLoadOrder_O275()
    {
        $o = new Order(275);
        $this->assertEquals(275, $o->OrderID);
    }

    public function testLoadOrder_O1_Fail(){
        $o2 = new Order(1);
        $this->assertEquals(1, $o2->OrderID);
    }
}

I have two other classes which are autoload using my configure.php, first is DB class that connect to db using mysqli extension and other is Order class that take int as input for construct and load it from database.

My first test succeed to load but second test gives

DB::Execute(): Couldn't fetch mysqli

Where DB is class name and Execute is my public function in that class [not static]. If I reverse the order of functions then first function start giving same error, but later works. Any reason? or solution?