PHP and security (like fopen)

Hello, this is my security problem with PHP have a free web hosting server and i permit users to use PHP and some functions like fopen ! The problem is that i would like to denied fopen to works with my own web file of my website users: /home/userxxxx/www/ Me: /var/www/html/ One solution consist to forbidden the user of fopen (i dont want this for their file ...) 2nd: all extension for my own php scripts are for exemple phx Can i accept the commande fopen with file php but forbid fopen("any_file.phx") ??? 3: Or, can i accept some function ONLY if they are execute from my own subnet ? The final alternative will be to modify directly the code source of php and recompile but any help will be welcoming ! Thanks !