mb_convert_case() returns case folded version of
string converted in the way specified by
mode can be one of MB_CASE_UPPER, MB_CASE_LOWER
encoding specifies the encoding of
str; if omitted, the internal character encoding
value will be used.
The return value is str with the appropriate case folding applied.
By contrast to the standard case folding functions such as strtolower() and strtoupper(), case folding is performed on the basis of the Unicode character properties. Thus the behaviour of this function is not affected by locale settings and it can convert any characters that have 'alphabetic' property, such as A-umlaut (Ä).
For more information about the Unicode properties, please see http://www.unicode.org/unicode/reports/tr21/.
Example 1. mb_convert_case() example