Official recommendation is to use cURL to trigger scheduled tasks, not executable PHP. Currently it is just a recommendation, but upcoming releases will stop shipping /tasks
folder so you will have to use cURL.
There are many environments (more than we expected) where there is one PHP that web server uses to prepare the page, and another PHP that runs via command line interface (CLI). This causes all sort of problems, so we decided to use only way way of triggering tasks - via URL.
Bottom line - use cURL. Documentation is here:
https://activecollab.com/help/books/self-hosted-edition/scheduled-tasks-setup.html
Here are sample commands:
*/3 * * * * /usr/bin/curl -s -L "http://url/of/frequently?code=XyZty" > /dev/null
0 * * * * /usr/bin/curl -s -L "http://url/of/hourly?code=XyZty" > /dev/null
0 12 * * * /usr/bin/curl -s -L "http://url/of/daily?code=XyZty" > /dev/null
0 7 * * * /usr/bin/curl -s -L "http://url/of/paper?code=XyZty" > /dev/null
but make sure to check Administration > Scheduled Tasks page of your activeCollab for exact URLs that you need to trigger.