How to make redirect if HTML-document doesn't exist on server

Question

How to make redirect to 404 or root page when user types query to not existing page and SSI include gets error?

Or how to check if HTML-document exist on server and if doesn't then also make redirect?

If user types domain.tld?anyquerystringon my site then it includes anyquerystring.html HTML-page by SSI.

If user types query of not existing page like domain.tld?blahblahblah then it gives the [an error occurred while processing this directive] in the place of SSI include.

Answer 1

You can configure error message in SSI. The answer is

<!--#config errmsg="Put here your error message, HTML or other code" -->

Answer 2

There is no way to check it with php, because if a page does not exists how your php code can run, you need to make changes to .htaccess file as it will tell the server to a redirect a user to a defined page in case of 404

RewriteEngine on
ErrorDocument 404 http://www.yoursite.com/404.php