On PHPMaster.com there's a tutorial posted that helps you understand how to work with multibyte strings in PHP. Multibyte strings could be a set of characters from a non-English language. They have to be treated differently than normal strings using the mbstring functionality.

A written language, whether it’s English, Japanese, or whatever else, consists of a number of characters, so an essential problem when working with a language digitally is to find a way to represent each character in a digital manner. Back in the day we only needed to represent English characters, but it’s a whole different ball game today and the result is a bewildering number of character encoding schemes used to represent the characters of many different languages. How does PHP relate to and deal with these different schemes?

He goes through a bit of introduction to multibyte strings - how they're represented internally, character schemes and Unicode. He also talks about the PHP support for the strings, noting that it's not really made to deal with them by default and the two methods you might use - iconv and mbstring. He shows how to enable the latter and introduces some of the most common functions you'll use with it (complete with some code examples).