Yes! There is a HUGE difference. Going through http:... generates an HTTP call which goes through the network and obtains the .php file. Going through / goes directly through the file system; this is much more efficient, more reliable, and more secure.
J wrote:
Is there a difference in calling the following include() path?
include("/home/sitename/public_html/config.php");
and
include("http://sitename.com/config.php");
?