Specifying Variable Document Path in PHP

Hey Folks, My first post to this list and forgive me if this is ground that's been trod before, but this has been bugging me for a bit. I'm developing a site that's going to be running on a staging environment and a production environment. Have a number of PHP includes in the production site that specifically call the file pathname (thus): <?php include '/home/production_site/public_html/pagename.php' ?> The problem here is that the path will vary slightly depending on whether it's the staging server or production server. I'd ideally like to set a single global variable that can handle this so I don't have to hard-code paths across the site (as I'm doing now). Is there any easy way to accomplish this? Thanks in advance, Anthony