I know Xampp very well, but i dont know Textwrangler. As this is a Xampp Forum (not a Textwrangler Forum, neither an OSX Forum), i have no idea how to answer your questions. I can use Xampp without Textwrangler and I dont know, how Textwrangler works in conjunction to Webservers. I dont think that someone may answer your questions here, as Textwrangler is pretty unknown to Xampp users. It is not part of Xampp.
P.S.: I can explain how to start your own PHP scripts in a browser, but this of course does not affect Textwrangler. You simply have to put your own PHP (and/or HTML) files into the htdocs folder, you may of course create a subfolder in htdocs and put your files there. Lets assume, your file is called test.php and you put it into the folder "myfolder", which has to be a subfolder of htdocs. In that case you have to enter http://localhost/myfolder/test.php
into your browser in order to see the output of your PHP test file.
If you simply call for http://localhost
in your browser, the so called "Xampp Dashboard" should appear. If not, something is going wrong with your Xampp installation.