"chrooting" php to public_html dirs

[dominant]

Hello

I have set a webserver for my users.

users have their dirs under the /home dir.

for example

/home/usera
/home/userb

Apache can access the public_html dir under each user's dir

for example

/home/usera/public_html
/home/userb/public_html

Now, every public_html directory is owned by wwwrun:www (unfortunately, this cannot be changed due to a policy)

So far, using php usera can acess userb's files just giving a relevant path (../../userb/public_html) (Safe_Mode is on)

It is obvious that this is a threat.
How could i prevent that php can see anything outside each user's public_html

Thanks in advance.