Suppose user directories are in:
/home
How to prevent the script
/home/john/script2.php
from opening
/home/george/script1.php
using fopen(), file(), etc.?

Just CHMOD it to something where nothing can access it.

OK, but the webserver runs with the same uid when parsing either files!!!

well you need to flag the file

$skipfile = "file1.php";
$dir = "path";

if (is_dir($dir)) {
   if ($dh = opendir($dir)) {
       // catch it in the while loop
       while (($file = readdir($dh)) !== false && $file = readdir($dh)) !== $skipfile) {
           or catch it down here
           if($file != $skipfile){
               echo "filename: $file : filetype: " . filetype($dir . $file) . "\n";
           }
       }
       closedir($dh);
   }
}

Of course I have not tested this so you may well have to tinker with it, but it should do the trick

ibbo

I think you've misunderstood me. It's about security mesures and configuration. The issue is not to allow users to open other users' files via PHP.