Debian permissions not sticking for new content

I've set up a VM with Debian7 for PHP development.

Since PHP needs to write to log files and the like, the www-data group needs write permissions on /var/www/...

I found a good answer here on superuser that makes it go real quick: https://superuser.com/a/19333

However, when I update a project with composer and new cache/log files are created, www-data does not have permission to write to them and I have to execute the last 2 commands again.

I was under the impression that setting permissions to 2775 meant that all future files would have the 775 permission? Or am I wrong about that? Is there a way to prevent having to reset the permissions every time or am I stuck having to do this?

Granted, it's not something that happens often, I'm just curious if there's a way to avoid it, other than setting up a cron job for it.

Answer

I think this is what you are looking for.
This answer describes how to use Access control lists (ACLs) for default permission settings for new files/folders, within a given folder. This is also part of the second answer of the link you provided, where Nikhil Chelliah states the following within his answer:

The setfacl command accepts -s to replace an existing ACL or -m to modify it; -R to make directory ACLs recursive; and -d to make the specified settings the default, which is useful if you're anticipating forthcoming user accounts.