Why does a "chmod 775" doesnt make a folder writeable by PHP? [on hold]

Why does a "chmod 775" doesnt make a folder writeable by PHP? [on hold]

My setup: A very simple "default" server:
fresh installed ubuntu server (12.04, 13, its always the same)
apt-get install apache2
apt-get install mysql-server
apt-get install php5 php5-mysql
My Problem:
to make a folder writeable by PHP I always have to do "chmod 777",
everything below simply doesn't work
My question:
Why does a "chmod 775" (etc) doesnt make a folder writeable by PHP ? I'm
asking from a coder's point of view to understand the ownership structure,
therefore I'm NOT asking on serverfault.com.