Certainly, this shouldn't be happening - included functions are supposed to be available. Could you supply some example code so's we can see what's happening?
This should certainly work:
----8< foo.php ------
function foo($bar)
{ ....blahblahblah....
return $baz;
}
-------->8-----------------
----8< wibble.php ------
include('foo.php');
...
$wibble = foo($blug);
-------->8-----------------