debian 6 - mkdir - permission denied

ispconfig: 3.0.5.2
debian: 6.0.7

vhost setting:
suexec
fastcgi

i have written a php script which will create a directory in /home. the php script is placed in web directory of vhost.

mkdir ("/home/xxx");

error: permission denied

/home: user/group = www-data

this works in debian 5.

pls advise.