How do I refer a file in php which is ahead of two folders

[thuanhung20]

I have the following structure \[code\] folder1 file1.php file2.php folder2 folder2.1(folder) file2.1.1(file)\[/code\]In the above structure how do I refer to file (file1.php) which in folder1 from file2.1.1.php which is in folder2.1