golive does not provide any php authoring/testing facilities (you could improve things a bit with zend studio and their golive plugin, but for an integrated web/php developmemt environment something like dreamweaver might be a better choice if you will be doing a lot of work)
but you can still use golive to manage the site...
to use php with golive you simply add the php files to your golive site, just drag & drop, or for a new file create the whatever.php file in golive and use golive's text editor to input your code
but golive won't
run php for you, so to actually test things properly you need to upload the site to a server supporting php
you can use your mac for this if you enable the webserver and set golive to ftp the site to the right place on the mac, you may need to tweak the apache config on your mac to get things working the same as your hosting service's
once set up, you can access the site from your mac as
http://localhost/
as another poster mentions, the adobe golive forum will give you a better audience for your question