In PHP, what does "<<<" represent?
例如：
$sql = <<<MySQL_QUERY
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.
This is called a heredoc, and it lets you do a long piece of text that goes over several lines. You can put PHP variables in there and they will replace with the value. The word CHART can be anything. It just needs to be the same to start and stop where the quoted text begins.
You could do the same thing by appending multiple quoted strings, but this is cleaner most of the time for extended documents like this HTML text. There is also something called a nowdoc which is like a single quote string in PHP, but these won't let you use variables inside them.
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