This is a short one for reproduction.

This one throws (SyntaxError: Parse Error : syntax error, expecting T_END_HEREDOC on line 4)

<?php echo <<<EOS  EOS. "";

This does not.

<?php echo <<<EOS  EOS . "";

Both are valid PHP code, with the same net effect. Though, the examples are basically as minimal as can be, to the point it would be nonsensical in real code. Replace "" with a variable and add some meat to heredoc, then you get something reasonable. Like something you might get in a test, like where I found this.

By the looks of it, it's something added in PHP 7.3, and was missed in #506, so should be easy to add to https://github.com/glayzzle/php-parser/blame/c702357f7435af363051a947b64d1e064cca014b/src/lexer/strings.js#L10 and have a test added.