It looks like this is going to be possible. Anyone with experience in php, please give me your opinion! This is an example of how to limit comments to only those who are members of your website. In this example I'm going to use phpBB members as they're integrated throughout my site. I haven't tested it yet, but this is what I've got so far.
*Note* If you're going to test this, back up every file you before you touch it. Like I said, I don't know if it will work. I will test it and post the result later.
This is the comment_form() function. From what I understand, it's called to display the comment form at the bottom of blog posts. It's located in wp-includes/comment-template.php on line 1509. Some changes will need to be made to it later.
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It looks like two files need to be changed in order to allow only the members of your site to comment. These two files are as follows:
1. wp-content/themes/ {Your Theme Name Here} /comments.php
2. wp-includes/comment-template.php
First things first, add the following to the top of comments.php. This is for phpBB users. If you use your own login system, include the proper files instead.
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It will need to be changed to this:
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Next up is comment-template.php. I don't think it's necessary, but go ahead and include the php for session management at the top of this file too. Then make the following changes to comment_form() (line 1509).
Find:
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Replace with:
[Code moderated as per the Forum Rules. Please use the pastebin]
These changes insert your users info into form. It also makes them hidden so changes cannot be made. If you want them to make changes, change type="hidden" to type="text". Also, a link is displayed in the value of the url field. This is a link to the users profile. You may need to change it depending on the location of profiles.
But, I've no idea if this will work. Just ideas. If anyone has experience with this, please comment on my idea! I would love the input/advice!
References for information:
http://codex.wordpress.org/Function_Reference/comment_form
http://devpress.com/blog/using-the-wordpress-comment-form/