How to set a directory inside public as class variable in laravel

I have a folder named source in my laravel 5.4 application which i use quiet often in my controller code.

For example the statement public_path() . '/source/' is used in many places in my code and so I want to define the public_path() . '/source/' directory as a controller level variable. This way I will just have to use the variable name in all controller functions that use the directory.

This is what I tried and was surprised to see it fail:

class PostController extends Controller
{
    public $source_dir = public_path() . '/source/';
}

But on the line where $source_dir is defined, the application gives the following error:

FatalErrorException syntax error, unexpected '(', expecting ',' or ';'

Thanks for any help!

via Chebli Mohamed