Why use multibyte string functions in PHP?

Here is my answer in plain English. A single Japanese and Chinese and Korean character take more than a single byte. Eg., a typical charactert say x is takes 1 byte in English it will take more than 1 byte in Japanese and Chinese and Korean. Now PHP's standard string functions are meant to treat a single character as 1 byte. So in case you are trying to do compare two Japanese or Chinese or Korean characters they will not work as expected. For example the length of "Hello World!" in Japanese or Chinese or Korean will have more than 12 bytes.

Read http://www.php.net/manual/en/intro.mbstring.php

All of the PHP string functions do not handle multibyte strings regardless of your operating system's locale. That is why you need to use the multibyte string functions.

From the Multibyte String Introduction:

When you manipulate (trim, split, splice, etc.) strings encoded in a multibyte encoding, you need to use special functions since two or more consecutive bytes may represent a single character in such encoding schemes. Otherwise, if you apply a non-multibyte-aware string function to the string, it probably fails to detect the beginning or ending of the multibyte character and ends up with a corrupted garbage string that most likely loses its original meaning.