Depending on what you need it for, a cleaner solution might be to make sure you serve consistent URL's first. To redirect all http://yoursite.com to http://www.yoursite.com put this in your .htaccess file (in root directory assuming your using apache).

    RewriteEngine On
    RewriteCond %{HTTP_HOST} ^your-site.com [NC]
    RewriteRule ^(.*)$ http://www.your-site.com/$1 [L,R=301]

More variations of this code [here][1]. This also helps a little with SEO, and is generally good practice. 


  [1]: https://stackoverflow.com/questions/234723/generic-htaccess-redirect-www-to-non-wwwhttps://stackoverflow.com/questions/234723/generic-htaccess-redirect-www-to-non-www