mb_strtolower() returns str with all alphabetic
characters converted to lowercase.
encoding specifies the
encoding of str; if
omitted, the internal character encoding value will be
used.
For more information about the Unicode properties,
please see
http://www.unicode.org/unicode/reports/tr21/.
By contrast to strtolower(), 'alphabetic' is
determined by 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 (ĩ.
See also strtolower(),
mb_strtoupper(),
mb_convert_case().