php将字符串全部转换成大写或者小写的方法

<?php
$string = "Learn PHP string functions at jb51.";
$lower = strtolower($string);
$upper = strtoupper($string);
print("$lower\n");
print("$upper\n");
print("\n");
?>

learn php string functions at jb51.
LEARN PHP STRING FUNCTIONS AT JB51.NET