I get "500 Internal Server Error" when opening install.php

Check your website error log for exact reason.

In case of log message "Invalid command php_flag": If you are having Red Hat Linux 8.0 default installation then it comes with faulty PHP version.

More details: http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=78586.

If this is not the reason, check if command "php_flag" is allowed for your website. Some hosting companies don't allow this by default.

php_flag in .htaccess file is not used anymore starting from Saurus version 4.3.0