cannot make and write text file with php on ubuntu

After some more research, I've got it figured out. Here is the solution for anyone having the same problem:

Since localhost belongs to the www-data group, I just added my user to that group.

sudo usermod -a -G www-data my_username

Then, I added the folder to the group.

sudo chgrp -R www-data /var/www

Then, I gave write permissions to the www-data group.

sudo chmod -R g+w /var/www

This worked for me without any other issue. Thanks!