activeCollab uses pretty strong assumption that all PHP files end with .php extension. There is no "one click" solution that lets you use other extension, but here are some tips if you want to try to make things work:

1. Change the Apache settings for the folder where activeCollab is working

Check this and this. Maybe it can help, I haven't tried it.

If you can't use .htaccess files or you are having problems with code provided in the post above contact your hosting provider and ask how to set up PHP5 to execute .php files in folder you wish. They can set that up for you in most cases, you just need to ask for that.

2. Rename all the files and change some source

Rename all activeCollab files from .php to .php5. Use some search and replace tool and replace '.php' with '.php5' in all activeCollab files. Try if it works. There is no guaranty that it will, but there should be no problems.

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Btw, if you have any other suggestions please reply.