PHP coding "global"

Hey,

I'm learning PHP from a book at the moment and am stuck with this bit of code:

//create function to get lead actor
function get_leadactor($lead_actor) {
global $actorname;
$query2="SELECT people_fullname
FROM people
WHERE people.people_id = $lead_actor";
$results=mysql_query($query2) or
die(mysql error());
$rows=mysql_fetch_array($results);
extract ($rows);
$actorname=$people_fullname;
}

I know that you are creating a function called "get_leadactor" and that it value will be stored in the variable "$lead_actor". What does "global $actorname;" do on the second line though?????

I've been doing searched for "global" on the internet but i just keep on getting the same info about turning regictered_globals on/off in the php.ini file, but nothing telling me what this line of code is doing.

Thanks for your help!!