PHP英文字母大小写转换如何实现

<?php
$foo = &#39;hello world!&#39;;
$foo = ucwords($foo);             // Hello World!
$bar = &#39;HELLO WORLD!&#39;;
$bar = ucwords($bar);             // HELLO WORLD!
$bar = ucwords(
strtolower
($bar)); // Hello World!
?>

<?php
$foo = &#39;hello world!&#39;;
$foo = ucfirst($foo);             // Hello world!
$bar = &#39;HELLO WORLD!&#39;;
$bar = ucfirst($bar);             // HELLO WORLD!
$bar = ucfirst(strtolower($bar)); // Hello world!
?>

<?php
$foo = &#39;HelloWorld&#39;;
$foo = lcfirst($foo);             // helloWorld
$bar = &#39;HELLO WORLD!&#39;;
$bar = lcfirst($bar);             // hELLO WORLD!
$bar = lcfirst(
strtoupper
($bar)); // hELLO WORLD!
?>