(I'm not exactly sure what you mean, but I'll give it a shot..)
If you declare a variable in a script, you can use it (regardless of
whether or not it's in the same PHP file)
The only time you need to specifically do something to use it
is when you are using a function.
Then, what you have to do is this :
$somevar = 'Hello there';
function test() {
global $somevar;
echo $somevar;
}
hope it helps