public function test_placeholders()
 {
     $qb = new QueryBuilder();
     $foo = $qb->addValue('foo');
     $bar = $qb->addValue('bar');
     $input = "this is {$foo} and {$bar} and {$foo} again";
     $expect = "this is ? and ? and ? again";
     $values = array('foo', 'bar', 'foo');
     $output = $qb->fixPlaceholders($input);
     $this->assertEquals($expect, $output[0]);
     $this->assertEquals($values, $output[1]);
 }