Hmm http://www.gnu.org/software/wget/ looks like it might be able to handle a recursive mirror of the site. (Not sure if it could rename the .php filenames to .html intelligently though)
You can create a PHP script to handle this, it is easier if only one directory though. You can use the opendir command to open the current directory, and then use $page = file_get_contents("http://yoursite.com/$entry") and then save the page as the page name of $entry minus 3 characters and add .html.