Update to not use `mb_` functions. Fixes #1

~~~ php
A valid class name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.
~~~

We can assume the `non-mb_` functions will be fine.

@seven1m is there a good way to test this? The current tests still pass and this is a type of refactoring. I can't think of a way to test this.