confused about getenv arguements

Hello, I found this script at zend. Please note the getenv("HTTP_REFFERER"). <? /* Anti-leech bandwidth protecter by Corey Milner, http://www.odey.com. Turn the refererring URL into a variable */ $from = getenv("HTTP_REFERER"); /* Check to see if the URL in the variable is a valid referrer. Add the page URL which you would like people to arrive from here. */ if ($from != "http://www.yoursite.com/validpage.htm") /* If the URL is valid, page loads now */ /* If URL is invalid the following error message and proper link appears, enter your custom error message and a hyperlink to the valid URL you entered above here*/ {print(" Sorry you have tried to link to a page which does not accept visitors directly. <br> <a href=http://www.yoursite.com/validpage.htm>CLICK HERE</a> to enter"); /* Prevent the rest of the page from loading */ exit;} ?> I went to check getenv in the php manual, and it said "You can see a list of all the environmental variables by using phpinfo(). You can find out what many of them mean by taking a look at the CGI specification, specifically the page on environmental variables. I made a php script phpinfo(INFO_ALL) to return everything for my host and did not see HTTP_REFFERER, not did I see any of the other args that were in the comments on the getenv page. I looked at the linked cgi pages and didn't find anything there either. If by not seeing these HTTP_like variables in my phpinfo, does that mean they are unavailable to me to use? Do they have to be setup during the install of php/apache? Where can I get more info on these getenv args/vars? Thanks