PHP treats all strings as ASCII by default. The recommended way of working with UTF-8 strings is to use the mb_string extension. Unfortunately this library is not always available. DokuWiki comes with a library that will handle all UTF-8 string in pure PHP when mb_string is not available.
Please refer to the API reference for available functions.
Note: Only use UTF-8 aware functions when needed. If operations can be done on byte level without special care for character boundaries this should be done as it is usually much faster.