无法在php中包含文件

->MainFolder
    ->folderA
        ->functionA.php

    ->folderB
        ->folderB2
            ->functionB.php

I trying to include "functionA.php" in my "functionB.php"

When i write "include('../../folderA/functionA.php')", the function inside "functionA.php" doesn't work in "functionB.php".

But the function works

when i write in "include('../folderA/functionA.php')"

I wonder why only up one level folder can run the function inside "functionA.php"