PHP - Relative paths "require"

My directory structure looks like this:

blog -> admin -> index.php
blog.php
db.php
functions.php

I have been trying to include (require, really) blog.php in the admin/index.php, but facing lots of errors. I'm following a PHP course, and the instructor does the same thing successfully.

admin/index.php:

require "../blog.php";

which, in turn, requires two more files in its directory.

require "db.php";
require "functions.php";