运行成功输出错误

<?php

    $my_book = "我的小写的book";

    $my_BOOK = "我的大写的BOOK";

echo $my_book;

echo "<br />";

echo $my_BOOK;

echo "<br />";

$my_book="这次奇迹要发生了";

echo $my_book;

echo "<br />";

?>