Multibyte-aware string function

I notice you used some emoji and therefore like unicode. I think it would be a good idea to (indirectly) teach users to use functions which can do international alphabets and emoji from the start.

I read through your guide and think it *is* awesome. In my decade as a PHP developer I have never seen such a succinct and informative explanation of abstract classes before. ❤️

chapters/functions.md

@@ -37,7 +37,7 @@ You can also return a value from a function. You can only
 return a single value from a function.
 ```php
 function capitalize($value) {
-    return strtoupper($value);
+    return mb_strtoupper($value);
 }
 ```