Using something like xxtest.php3 it does not seem to 
recognize/acknowledge the constant definitons, the
return value of define shows ok but when the constant
actually gets used it does not become translated,
instead one gets the constant name (XXTOP). I tried 
infinite times without success, even restarted the
web server. At some point, all of a sudden it started
working. I implemented then something else following
the same approach and then again the original was
working but the 'cloned' one was not working.

The same happened when trying to load the
define'd constants from a file with include(). And
in this case require() would not even work.

--- xxpaths.php3 ---
function xxPaths() {
    global $SERVER_NAME;
    if ($SERVER_NAME == "www.production.com") {
      $home = "/usr/home/someone";
      $whome = "$home/www";
      $top = $SERVER_NAME;
   } else {
      $home = "/home/someone";
      $whome = "$home/public_html";
      $top = "homehost.isp.com/~someone";
   }
   define("XXTOP", "http://$top");
   define("XXHOME", "$home");
  define("XXWHOME', "$whome");
}
----- xxtest.php3 ------
include("xxpaths.php3");
xxPaths();
print( XXTOP . "/php3lib");