How to set up default permission and group

[Mik J]

Hello,
I want one for one of my user this behavior.Each time he creates a file it 
needs to have permission/owner of rw-rw---- myuser www
This user is executing a php script that creates a file and I want that file to 
be read/write access by the user wwwI don't want to touch to the php script.
What is the good way to do that ?
Regarding the group ? Should I make www the main group of myuser ?
Thank you