Catching object instantiation error

Hello,

Hope someone out there can help me.

I am trying to catch error or prevent from instantiate a non existing
object class. I use:

class A
{
  ...
}

$className = "A";
$obj = new $className;
 ...
$className = "B";
$obj = new $className;


I would like to catch the error in the second part of the code, is there
a function to check if the class definition exist before the
instantiation?

Thanks,
Ly Vuong