To resolve a PHP error related to an empty string in the URL caused by an htaccess problem, you have to take the following steps:

1. Check Your htaccess File:

Ensure that your .htaccess file is properly configured and does not contain any rules that conflict with your URL structure. Look for any directives or conditions that may be causing issues, especially those related to empty strings.

2. Handle Empty Strings in PHP:

If your PHP code encounters an empty string in the URL and it's causing errors, add a check in your PHP script to handle this scenario gracefully. You can use conditional statements to verify whether the required parameters are present in the URL before processing them.

3. URL Rewriting in htaccess:

If you are using URL rewriting in your htaccess file, ensure that it is correctly configured. Make sure the rules are not unintentionally transforming empty strings into unexpected values or causing redirection issues.

Now to rewrite to 404.php if users enter anything after example.com/ you can check THE_REQUEST based condition. THE_REQUEST variable represents the original request received by Apache from your browser and it doesn't get overwritten after the execution of other rewrite directives. The example value of this variable is GET /index.php?id=123 HTTP/1.1

So combining it all, your full suggested .htaccess should look like this:

However, do keep in mind that last rule will also redirect any request for resources such as images, stylesheets and js files to >https://example.com

In case of error, use PHP error reporting to identify the exact error messages and warnings. You can enable error reporting at the beginning of your PHP script or in your PHP configuration.

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