Using a variable to dynamically load a class

I have decided to use the singleton pattern for my application. It makes the most sense to me. However, just when I feel like I have made some progress I run into another wall. I have a load function. The load function does the following. Check if class has been previously loaded. - If so - return $class::get_instance(); - Otherwise - look for class in various places - if found - return $class::get_instance(); - else return error. Before adopting the Singleton pattern I was instantiating classes with the load class. In the controller I would have this. ``` $session = $this->load->library('session'); ``` The load class would then find the file and return.. ``` return new $class_name; ``` I hoped that the in changing the method of loading classes it would be a tweak to a few lines but these tweaks are generating syntax errors. ``` return $class_name::get_instance(); ``` Is there a way to write the line above without the syntax error?