In apache, I used to handle 404.php by $url = getenv("REQUEST_URI"); to get the requested page. Thus, if someone redirected to 404.php from /test.html, I had a php string containing "test.hetml". This is indeed due to the mechanism of redirecting by "ErrorDocument 404 /404.php" in .htaccess

In Nginx, I think the mechanism for "error_page 404 /404.php;" is different. How I can get the requested page in 404.php ?