Error

Fatal error: Class comment_admin: Cannot inherit from undefined class comment in class_comment_admin.php on line 18

Just include the comment class in the same page before the comment_admin class is included.
The comment_admin extends comment, but does not include it. This has some reasons but my english is not so well to describe my feelings about inherited classes that include other files.

Can you give an example of what the page coding would look like?

include 'includes/class_comment.php'; // <-- comment_admin needs this one
include 'includes/class_comment_admin.php';

$comment_admin = new comment_admin($sqltable, 'css/'.$style.'/' );
$comment_admin->code_for = $admin_code_for;
$comment_admin->id_for = $admin_id_for;

...