I'm not sure I understand correctly. You want to take a normal static HTML file and name it something.php? There will definitely NOT be any benefit to rankings. There is no sense in doing so, since if the file is .php, the server has to parse it (looking for php code). This puts an unnecessary load on the server.
if you really wan't to make it seem like you have .php, throw in a .htaccess file, you can use it to either make .php redirect to .htm or make .htm parse PHP code
going through and renaming all files and changing links is too much work, of course if all file permissions are set to 0777 you can write a php file to go through and change all of the .htm(l) to .php in the source and then rename the file, but this is a lot of unnecessary work