PHP: C extension has undocumented "append" function that has no package parity.

[SpencerMalone]

What version of protobuf and what language are you using?
Version: Any/all. Has existed from the introduction of the package
Language: PHP

What did you do?
https://github.com/protocolbuffers/protobuf/blob/main/php/ext/google/protobuf/array.c#L245-L263

RepeatedField and the now removed Internal\RepeatedField in the C extension have an append() function that has never had a matching append() function in the php-only package, which can cause static analysis and type confusion.

[PilgrimMemoirs]

Thanks for catching this, we will accept a PR for it.