Create personal page for every user, PHP

[yayamas]

What i want to do is to create an web page for every user when they signup on my page. Example: www.someweb.com/username and this will be their webpage. I know that this can be done easy with mkdir() and other related functions, but the problem is that my root folder is not chmod 777 and i don want to chmod 777 this root folder because of security reason. What is the best way to do this thing when user registers on my web page