Variable objects?

I'm planning to use variable objects in a new project I'm working on. The problem is, I can't find one page of documentation on them, so I can't be sure if I'm going to be using an accidential feature that will disappear. As an example of them working, the following outputs "In a_class.": <?php class a_class{ function a_class(){

       echo 'In a_class.';

} } $foo = 'a_class'; new $foo(); ?> -- The above message is encrypted with double rot13 encoding. Any unauthorized attempt to decrypt it will be prosecuted to the full extent of the law.