How to delete a file via PHP?

How do I delete a file from my server with PHP if the file is in another directory?

Here is my page layout:

• projects/backend/removeProjectData.php (this file deletes all my entries for the database and should also delete the related file)
• public_files/22.pdf (the place where the file is located.)

I’m using the unlink function:

unlink('../../public_files/' . $fileName);

But this always gives me an error that the file does not exist. Any ideas?