Can I write custom functions in PHP member methods?

Can I write custom functions in PHP member methods?
I defined a function in a method, but an error was reported when calling it
This is the method that reported the error

The error was reported as follows

But I have another method that also writes functions and performs recursion, but no error is reported.

This is the interface for calling these two methods

Why is an error reported? Can anyone explain it or have a good way to deal with it