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
Most likely you don't need to create these directories in real.
Just make it virtual.
Pass a username using query string, like this:
www.someweb.com/index.php?user=username
And personalize this page according to particular username.
After that you can do some rewrite magic and make a page address like this www.someweb.com/username but all pages will remain virtual
Use mod_rewrite to make a request to /username actually be ?user=username. You can then get the appropriate user's data and display it in a template.
You don't make physical directories for each user, you use URL rewriting. Take a look at
http://www.yourhtmlsource.com/sitemanagement/urlrewriting.html