Using a .php5 extension

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On my host, in order to use PHP5, I have to have the extension php5 on the index.php file. When I rename it index.php5, it seems to think all the includes or whatever should be php5 also. For example, I get an error about not finding system/codeigniter/CodeIgniter.php5. If I rename that, it complains about another.

I expect I could simply rename everything with a php5 extension and it would work, but I hope there is a better (more expected) way of handling this that would upgrade more easily among other things.

Anybody know how to do this? Am I simply not finding a config option in the code or documentation?