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 (Ä).
Examples
Example #1 mb_strtolower() example
<?php $str = "Mary Had A Little Lamb and She LOVED It So"; $str = mb_strtolower($str); echo $str; // Prints mary had a little lamb and she loved it so ?>
Example #2 mb_strtolower() example with non-Latin UTF-8 text