It's what is called a heredoc.
Heredoc
A third way to delimit strings is the heredoc syntax: <<<. After this
operator, an identifier is provided, then a newline. The string itself
follows, and then the same identifier again to close the quotation.
The closing identifier must begin in the first column of the line.
Also, the identifier must follow the same naming rules as any other
label in PHP: it must contain only alphanumeric characters and
underscores, and must start with a non-digit character or underscore.
Reference: http://www.php.net/manual/en/language.types.string.php#language.types.string.syntax.heredoc