PHP8 compatibility needs just a simple patch.

% start.php @line 154
foreach (array ('users/', 'lib/', 'themes/', 'cgi-bin/') as $_) if (stripos ($_, PATH) === 0) die ();
%

change to:

% start.php @line 154
if (PATH != "") foreach (array ('users/', 'lib/', 'themes/', 'cgi-bin/') as $_) if (stripos ($_, PATH) === 0) die ();
%

and also:

% lib/domtemplate/domtemplate.php @line 23
class DOMTemplate extends DOMTemplateNode {
...
}
%

move down this whole class definition block after that one:

% lib/domtemplate/domtemplate.php
abstract class DOMTemplateNode {
...
}
%

The new location for the DOMTemplate class becomes around line 380.

:: @Pierre-Marie Baty added on 05 May ’22 · 14:06

*edit* I almost forgot one.

% lib/utf8safe.php @line 50
if (get_magic_quotes_gpc ()) {
%

change to:

% lib/utf8safe.php @line 50
if (function_exists ("get_magic_quotes_gpc") && get_magic_quotes_gpc ()) {
%

After that my copy seems to work (nginx-1.20.2/PHP-8.0.18).