How to check if a library exist in a controller?

PHP Code:

if(file_exists("../libraries/myapp/" . ucfirst($library_name).".php")) { 


This code is not working for me.
It is written inside of a controller.

I have a default library and some special libraries.
If special libraries are just the controller names.

If they exist, they should be loaded. Or the default one should be loaded.

PHP Code:

if(file_exists("../libraries/myapp/" . ucfirst($library_name).".php")) {
    $this->load->library("myapp/".$library_name);
}
else {
    $this->load->library("app_default_library");
} 


What is the right way to check the existence of a library?