这是HEREDOC syntax，

用于定义多行的字符串。

Heredoc text behaves just like a double-quoted string, without the double quotes. This means that quotes in a heredoc do not need to be escaped, but the escape codes listed above can still be used. Variables are expanded, but the same care must be taken when expressing complex variables inside a heredoc as with strings.

$a = 'World';
$string = <<<MARKER
<p>
  Hello, $a!
</p>
MARKER;
echo $string;

将输出：

<p>
  Hello, World!
</p>