Make it easier to use local variables and defined values

One thing I struggle with often is the difficulty in working with the GUI and adding expressions, local variables, and defined values.

I often have to add them to an array WA will recognize like POST or GET, but I have to worry about whether an extension will read that array and forward that information such as passing all of the get array as a query string when I don't want that.

I have taken to adding the values to the COOKIE array without actually setting the cookie, but I would really like to add local variables like $myredirect or defined values like SSL_SERVER [ from define("SSL_SERVER","http://my.ssl.server") so I can put that in a config file].

And a tutorial or flexibility to add expressions easily. I was just trying to make a change in the PayPal Express checkout behavior to go from:

<?php echo ("http://".(isset($_SERVER["SERVER_NAME"]))?$_SERVER["SERVER_NAME"]:"")."/test.php?txn=complete"; ?>

to

<?php echo (!in_array($_SERVER['SERVER_ADDR'],$exempt_addr)?"https://":"http://").((isset($_SERVER["SERVER_NAME"]))?$_SERVER["SERVER_NAME"]:"")."/test.php?txn=complete"; ?>

I think it didn't like $exempt_addr, so the GUI accepted my input, but the code came out:

"".((isset($_SERVER["SERVER_NAME"]))?$_SERVER["SERVER_NAME"]:"")  .""