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Why?

Have you ever left a comment on a blog, only to forget about it and never come back? Thank Me Later is great for your blog because it reminds visitors of their comment a few days later, and prompts them to engage in further discussion and reading on your blog. It gives you the opportunity to keep alive a connection with the visitor after they have left your blog.

Is this not a bit invasive/annoying?

It can be, but selecting the correct options in TML will ensure it is not. For example, you can configure TML to only ever send one e-mail to each visitor, or leave a gap between sending e-mails. This will ensure you do not annoy those who actively engage with your blog.

How do I use PHP in e-mails?

Any text enclosed in <?php and ?> (PHP code delimiters) will be treated as PHP code. You can use the variable $comment_id to obtain the ID of the current comment, and use this to obtain, process and output content in e-mails. You can also use some third-party plugins to insert content into e-mails.

Here are some examples of code:

1. <?php echo "Your comment ID is $comment_id"; ?>
2. `Your comment: "comment_content, 0, 30)."..."; ?>"
3. You can find your comment at this URL: comment_post_ID)."#comment-".$comment_id; ?>

More

Ask lots of questions: http://blog.pipvertise.co.uk/wordpress-plugin-thank-me-later/