adding and editing non-required field in comments

It’s OK to post updates to you progress. I can tell you’re excited to be making progress and to have some of this coding stuff starting to sink in 🙂

Remember that comment author link output occurs during a GET request, so there will be nothing in the global $_POST. The author link function is where
if ( isset( $webname ) && '' != $webname ) {
comes into play. It was wrong for saving meta, but it’s right in this situation. If that resolves to true, then you append " ($webname)" to $author before returning the link HTML. There is no code for the false or else condition because there is nothing special to do. Of course the function always does return $author; at the end no matter what.

BTW, when you append data from the DB that will be output, it’s customary to pass the data through an escape function such as esc_html(). This protects your visitors in the off chance your site gets hacked and it inadvertently contains malicious code in the data. $author .= esc_html(" ($webname)");

Heh, I was trying to get you to work out the final solution to the parentheses quandary by dropping a bunch of hints. One usually learns more that way… or fails. Failure is an option, it’s OK. Second best is stewing on the problem for a while, then peeking at the answers. This ought to do what you want if the no answer state stores an empty string:

function attach_websitename_to_author( $author ) {
    $webname = get_comment_meta( get_comment_ID(), 'webname', true );
    if ( isset( $webname ) && '' != $webname ) {
        $author .= esc_html(" ($webname)");
    }
    return $author;
 }

But now it seems your DB has several variants of the no answer state, from no record to empty string, space character, or  . Ideally, you ought to update the DB to be consistent. This can be done with phpMyAdmin if you are very careful and first make a backup of the post meta table for safety’s sake, and if there are not too many records to change. Manual editing is relatively safe as long as you stick to only changing the “webname” fields one at a time. It’s possible to change a bunch at one time, but doing so is dangerous unless you are confident with SQL queries.

If you’d rather not make your DB consistent, we can code for all the possible variants by using the in_array() function, one of my favorites:
if ( isset( $webname ) && ! in_array( $webname, ['', ' ', ' '])) {

isset() is a legitimate function, but doing !isset( $webname ); on its own doesn’t accomplish anything because the return value is ignored. It’s normally used in conditional statements where the return determines which conditional code to execute. Example:

if ( isset( $webname )) {
  echo '$webname is set';
} else {
  echo '$webname is NOT set';
}

When we use isset() like this:
if ( isset( $webname ) && '' != $webname ) {
it serves to prevent unassigned variable warnings when we do '' != $webname should $webname not be set for any reason.

Also, if $webname = ”, is it not possible to display the emptiness with emptiness, rather than a tilde (or as before, inside parentheses)?

You can display whatever you like if it’s coded right 🙂
Not appending anything to the author link is the same as displaying emptiness, right? That’s what my “answer” code above does. You could add an else condition and append blank spaces, but browsers normally ignore extra spaces, so there’s little point in doing so.

Since you standardized the DB entries, there’s little reason to use the in_array() right now anyway. Stash it in the back of your mind as a tidy alternative to a series if-x-or-y-or-z-or… statements.

Getting mixed up with this stuff is par for the course when starting out. When you keep at it it starts to make sense eventually. Needing to make sense of something never really goes away. I still regularly use a special page I built just for testing out code snippets I create to help me understand something. That thing has taught me a lot!

The parentheses is not managed when data is saved, it’s done upon output. Look at the attach_websitename_to_author() function to manage the appearance of parentheses.

Your latest “comment_post” callback is not addressing the condition where nothing is submitted or when you try to clear the field. Also, you can strip out extraneous whitespace with the trim() function. This way your code to reduce non-content data to an empty string will be more robust. What if someone entered "   "? It’s hard to predict all possible whitespace strings someone might enter, but using clean up functions like trim and kses handles it regardless.

You have if (($_POST[ 'webname' ] = ' ').... A single = sign in an if conditional is a red flag to me. At times it’s legitimate, but usually it’s a mistake. A single = is an assignment operator. You are assigning the value ' ' to the webname value in $_POST, destroying the originally submitted value. You want to use the double ==, the comparison operator. Nothing is assigned, the two sides are merely compared to see if they have equivalent values.

Your function logic for saving the webname value should be more like this:

if ( isset( $_POST['webname'])) {
   $webname = trim( html_entity_decode( wp_filter_nohtml_kses( $_POST[webname])), " \t\n\r\0\x0B\xC2\xA0");
} else {
   $webname = ''
}
add_comment_meta( $comment_id, 'webname', $webname );

html_entity_decode() converts   to the actual non-breaking space character. The " \t\n\r\0\x0B\xC2\xA0" argument for trim() lists all the whitespace characters that should be trimmed from the start or end of strings. \xC2\xA0 is the actual UTF-8 encoding for the non-breaking space character.

Argh! Sorry about the missing ;. It’s a frequent recurrence for me, I can’t seem to remember to end statements with it. I’ve a lot of experience in languages that do not use it, but a fair bit with those that do, so I don’t know what my hangup with ; is. Good job sleuthing out the error though. As you discovered, the error messages related to such an error are misleading.

Good plan to take a break when stuck on something. Sometimes a new thought comes of it, sometimes not, but a break is always good, especially a healthy break like a ride. I took “bike” to mean cycling, but motor biking for pleasure is at least a good mental health exercise.

I’m unclear what the issue is with   still. It shouldn’t be possible to save it with new entries after running data through trim( html_entity_decode()), and I thought they were cleaned out of the existing data.