heredoc没有按应有的方式缩进或工作

PHP新手。尽管在语法方面严格遵循了这本书，但在heredoc上还是遇到了一些令人沮丧的问题。下面的文本没有按应有的方式缩进。

<?php
$text="Mike's";
echo <<<_END
<!--END is just like double quoteing a var..
You can use single/double quotes without having to escape them first; inside 
END. The last _END tag, has to be on the start of new line with nothing allowed
to procede it, not even whitespace-->
This is the $text 'first line'.
This is the $text 'second line'.
This is the $text 'third line'.
_END;
?>