PHP: maximum execution time exceeded – fix

What the message looks like

PHP Fatal error:  Maximum execution time of 30 seconds exceeded in /var/www/html/wp-admin/includes/class-wp-upgrader.php on line 214

A slow database query, a slow external API or a large import or export.
The task is simply bigger than the default limit: updates, backups, image batches.
A loop that never ends or waits for something that does not happen.
The limit was raised for PHP, but nginx or a proxy still cuts the request earlier (then you see 504 instead).
The command line has no limit (default 0), so the problem only shows in the browser.

What limit does PHP-FPM use?
grep -rn 'max_execution_time' /etc/php/*/fpm/
Check the active value from the web server's point of view
php-fpm8.3 -i | grep -i max_execution_time
Is the web server timeout shorter?
nginx -T 2>/dev/null | grep -E 'fastcgi_read_timeout|proxy_read_timeout'

Find the slow part (query, API call) and speed it up. That is the real fix.
Raise the limit where needed: max_execution_time = 120 in the FPM php.ini, or php_admin_value[max_execution_time] = 120 in the pool, then reload PHP-FPM.
Raise fastcgi_read_timeout in nginx to match, otherwise users get a 504 first.
Run long tasks as a cron job, queue worker or CLI command instead of in a web request.
In your own code, call set_time_limit() only for the one script that needs it.

nginx has its own limit, 60 seconds by default. See upstream timed out (110).

A long limit lets one stuck request block a worker for a long time. Keep it as low as the real task allows.