大文字のアルファベットなどを小文字に出来るかどうか
例）
<?php
$str = "Mary Had A Little Lamb and She LOVED It So";
$str2 = 'ＭＮＬ';
echo strtolower($str).PHP_EOL;// mary had a little lamb and she loved it so
echo mb_strtolower($str).PHP_EOL;// mary had a little lamb and she loved it so
echo strtolower($str2).PHP_EOL;// ＭＮＬ
echo mb_strtolower($str2).PHP_EOL; //ｍｎｌ
strtoupper()とmb_strtoupperも考え方は同じ