Proper security settings for virtual hosting of domains?

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http://www.linuxquestions.org – I have a CentOS 5 server in which I use Virtual Hosting and each domain has its own user/pass for login to upload files. The path is /var/www/vhosts/[domain name]/httpdocs/ What im attempting is setting up the creation of the [domain name] folder from an administration backend under PHP, which I am developing. What Im worried about is if I allow PHP to run command line commands such as mkdir, then what is stopping anyone from doing the same from their php files on my server??? What is the best way to properly setup my server to allow automated creation of the domain structure within my folder system? I hope I explained that well enough.. if not, please inquire for further answers. (HowTos)