[Policy, no patch] Normalize on usage of is_callable() instead of function_exists()

I agree, this is a good thing to do. Can we use phpstan to only allow the is_callable function?

There is agreement here on making this change. And, in fact, there have been several issues implementing the change.

A policy decision should be accompanied with documentation. And there has been a suggestion to enforce this change. That implies this could be a coding standard. Although forbidding the use of a PHP function seems too string.

I have created a separate issue for doing the actual conversion, 📌 Convert function_exists() to is_callable() Active . Anyone wanting to discuss making this a coding standard should open an issue in the Coding Standards project.

With all that I think the work on this part is complete.