well, this is the best solution. The only thing is now we know kyu should be related to languages ! :)

Technicaly, the requirement didnt say EXPLICITLY that we could not use built-in function so there is a gray area. So despite this aproach is not "handmade from scracth" is also valid acording the kata definition.

I write my code in a complicated way because I didnt know simpler function.
this is the benefit of practices, you learn what you dont know and it will be added to your experiences.

PHP is strong :)

Yes, this only works if the string starts with C and ends in m.
Plus it won't work if there's anything in between besides '.'
Not the best solution for sure but it works nice.

Your goal in this kata is to implement a difference function

I mean...

What is the point of using inbuilt methods?
How is it going to help to improve your coding skills?

That so useless ...

It's just cheat ...

(but it works yes ....)