In PHP, what does "<<<" represent?

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.

Example:

echo <<<HEREDOC
This is a heredoc string.

Newlines and everything else is preserved.
HEREDOC;

It is the start of a string that uses the HEREDOC syntax.

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.