No, this is not exactly correct.
Yes, you can access public static method of one class from another class but you still need to include that class using any of the include techniques: require or include or rely of autoloader function to find and include that class for you.
File containing that class has to be included one way or the other.
Remember in php you can have multiple classes per file, it's up to you. Usually it's a good design practice to write one class per file, but this is not Java, and you can even write all your project's classes in one very big file.