Check if index in $args isset / ! empty before using it in Walker_Comment methods or create default $args in case if it is empty

[oglekler]

Taking into account the comments in #53449, I suggest local fix for the comments walker (wp-includes/class-walker-comment.php).

Class Walker_Comment uses some $args indexes without checking if they exist. And in my case, they don't exist, $args are empty array and I am getting notices. I will consider adding $args into the call, but they are commented as optional and don't suppose to be used in such way.

Examples:

public function start_el uses:

$args['short_ping']
$args['format']

And

$args['style']

is used without checking that it exists in several methods - end_el, end_lvl, ping, comment, html5_comment, start_lvl.

[SergeyBiryukov]

Hi there, thanks for the ticket!

It looks like the Walker_Comment class assumes that all the default arguments of wp_list_comments() would always be available, which may not necessarily be a correct assumption, so checking if they exist seems like a good idea.

To help move the ticket forward, could you share the steps to reproduce the issue on a clean install?

I added default values to each methods from Walker_Comment using $args to prevent using values without being sure they were initialized.

Trac ticket: https://core.trac.wordpress.org/ticket/56539

[sajib1223]

Steps to reproduce the issue are needed to investigate further.

## Trac Ticket

https://core.trac.wordpress.org/ticket/56539

## Summary

Walker_Comment methods access $args keys (style, short_ping, format, avatar_size, max_depth) without verifying they exist. When called with an empty $args array (which is the documented default), this produces PHP warnings on PHP 8.0+.

## Changes

• Added wp_parse_args() with defaults matching wp_list_comments() at the top of each affected method: start_lvl, end_lvl, start_el, end_el, ping, comment, html5_comment.
• Added PHPUnit tests to verify no warnings when methods are called with empty args.

## Notes

This is a cleaner alternative to PR #4126, which mixes the fix with unrelated whitespace/indentation changes. This PR only adds the defaults with no other modifications.

[alexodiy]

Test Info & Reproduction Report

Steps to Reproduce

1. Enable WP_DEBUG and WP_DEBUG_DISPLAY in wp-config.php.
2. Have a post with at least one comment (default sample data works).
3. Call Walker_Comment directly with empty $args:

$comments = get_comments( array( 'number' => 1 ) );
$walker   = new Walker_Comment();
$output   = $walker->walk( $comments, -1, array() );

Actual Results (PHP 8.1)

Warning: Undefined array key "format" in class-walker-comment.php on line 196
Warning: Undefined array key "style" in class-walker-comment.php on lines 237, 300, 318, 392
Warning: Undefined array key "avatar_size" in class-walker-comment.php on lines 323, 324
Warning: Undefined array key "max_depth" in class-walker-comment.php on lines 371, 384

Methods start_lvl, end_lvl, start_el, end_el, ping, comment, html5_comment all access $args keys without isset(). The PHPDoc documents $args as optional with default array(), so this is a valid call.

Why a New Patch

PR #4126 has been open since 2022 with no merge activity. It also has a few issues:

• Mixes the actual fix with unrelated whitespace and indentation changes, making review harder.
• Uses array_merge() for defaults instead of wp_parse_args(), which is the standard WordPress approach.
• Does not include PHPUnit tests.

I submitted a cleaner alternative: ​PR #11110

What the Patch Does

• Adds wp_parse_args() with sensible defaults at the top of each affected method: start_lvl, end_lvl, start_el, end_el, ping, comment, html5_comment.
• Default values match wp_list_comments(): style => 'ul', avatar_size => 32, format => 'xhtml', short_ping => false, max_depth => ''.
• No whitespace or indentation changes — only the fix itself.
• Includes 5 PHPUnit tests covering all affected methods with empty args.

All 9 warnings resolved after applying the patch.

Environment

• WordPress: 7.0-beta2 (trunk)
• PHP: 8.1.10
• Server: Apache
• OS: Windows 10
• Theme: Twenty Twenty-Five

cc @oglekler