如何访问文件夹的根目录

recently i worked in a project. the project has one root directory named "Project".the "Project" directory has also two sub-directory named "project_a" and "project_b". the "project_b" directory has two more sub-directory named "project_b_1" and "project_b_2". when i am on a page which is contains "project_b_2" directory then how can i access a file which is exists in the "project_a" directory. all code must be php code.recently i worked in a project. the project has