I'm porting an old project to symfony2 in order to start learning the framework and I cannot realize how to properly add some helper functions I have in a PHP file.
Services. Wrap those function in a class (like Helper), define the class as a service and then inject it where needed (controller or another service, or even into a cli command).
The namespace (thus the path, as you tagged the question with psr-0) is up to you.