MyBB Community Forums

If setting AdminCP / Configuration / Forum Display Options / Users Browsing This Forum is set to Off:
Warning [2] Undefined variable $usersbrowsing - Line: 22 - File: forumdisplay.php(1563) : eval()'d code PHP 8.0.28 (Linux)

Fix by adding new line at 278 in forumdisplay.php
$usersbrowsing = '';

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Similar when AdminCP / Configuration / Show Thread Options / Users Browsing This Forum is set to Off:
Warning [2] Undefined variable $usersbrowsing - Line: 97 - File: showthread.php(1617) : eval()'d code PHP 8.0.28 (Linux)

Fix by adding new line at 1527 in showthread.php
$usersbrowsing = '';

Been a few of these where features that are enabled 99.9% of the time don't have variables defined when they're disabled.