这是heredoc语法.您可以通过在字符串中添加<<<以及您选择的标记来开始heredoc字符串，并通过仅在新行中放置该标记(而不能做其他任何事情！)来终止它.为方便起见，有一个例外:允许您在结束定界符后添加一个分号.

That's heredoc syntax. You start a heredoc string by putting <<< plus a token of your choice, and terminate it by putting only the token (and nothing else!) on a new line. As a convenience, there is one exception: you are allowed to add a single semicolon after the end delimiter.

示例:

echo <<<HEREDOC
This is a heredoc string.

Newlines and everything else is preserved.
HEREDOC;