我想调用一个类对象，作为给定参数的类名

I want to do something like this: (in php)

$a = "class_name1";    
$b = "class_name2";
$object1 = new $a;
$object2 = new $b

is this possible?

Yes:

class A {
   public function foo(){
       echo 'bar';
   } 
}

$a = 'A';
$object = new $a();
$object->foo();

outputs

bar

You can test such things by yourself very fast on codepad.