Most PhD programs involve some classes at the beginning - especially programs which don't require a master's degree first. The classes are graduate-level progressions from similar undergraduate classes. Typically, PhD research goes on more slowly during the first year or two of the program while classes are taken, and the dissertation research topic is not completely fixed. For example, a PhD student planning to research programming languages would take the most advanced language classes available, plus math and optimization classes, for at least a year while doing preliminary research before choosing a dissertation subject.