Custom functions were introduced recently, with a restriction that non-custom functions could not be replaced. However, reviewing a recent issue has led me to the conclusion that it might sometimes be impractical to implement an Excel function in PhpSpreadsheet for all users, but it could still be helpful to some users to offer an implementation anyhow.
The Excel ASC function is currently unimplemented in PhpSpreadsheet, and is a candidate for replacement using the functionality added in this PR. This PR supersedes PR #4513, which I will now close, and its earlier incarnation PR #4511. For reasons discussed in 4513, I don't see a way forward for implementing the Excel ASC function in a way that would be generally usable. However, this PR would permit a user to implement ASC in a way which would satisfy the user's local requirements. See the new `testReplaceDummyFunction`; I think, or at least hope, that, despite the deficiencies of 4511 and 4513, this might satisfy the author's requirement.
For an overview of why this is desired (and ways that people have coped with its absence), see issue #2900 and issue #4048; also PR #4043 which will be superseded by this PR.
The list of Excel functions is moved from Calculation/Calculation to its own member. I believe that it is done in a way that will not cause big complications to two experiments from @MarkBaker (PR #2714 and PR #2734). I believe it is also done in such a way that further refactoring of Calculation can follow this model.
Custom functions can be added or removed from the function list. You cannot add a function if it already exists in the list, and you cannot remove a non-custom function from the list. They will, of course, not be understood by Excel if written to a spreadsheet; the use case is mainly using the Calculation engine outside of spreadsheet context.
oleibman