Creating a re-direct for 404 pages - php

Question

I created a 404.php page and added the following in my .htaccess file
ErrorDocument 404 /404.php
Also played around with removing the slash, adding a full URL, etc,
No matter what I do, I get my index.php UI regardless to what I write in the URL. Here is the thing, IT IS NOT re-directing me to domain.com/index.php or "/". The URL remains, but the UI is the index.php content
I'm adding my re-direct below in case you see something there that is conflicting
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www.example.com$ [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [L,R=301]
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Answer 1

From what I know of the mod_rewrite the last 4 lines are saying
That if the request filename is not a file nor a directory,
redirect to index.php.
And this is what is happening I believe.
All of the routes that are not available on the physical path in your application are going to index.php.
You will need some 404 mechanism in your application. If you are using some framework it usually has an exception like "RouteNotFound" thrown and then error handler redirects to a 404 page, you can also do something similar and redirect from within your application to 404.php.
if($should_be_an_error) {
header("Location: 404.php");
}
(haven't used something like this for years but should be something similar)
or remove
`
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
`
But then your other routes might stop working.
EDIT:
Since you wanted to redirect to 404.php instead of index.php(getting that from comments) the code should be changed to
RewriteRule . /404.php [L]
(last line of the rewrite block)

Answer 2

You can do this using FallbackResource directive. Remove all the code starting with RewriteBase / line and use this single line:
FallbackResource /404.php
This will fallback to /404.php for any 404 event.

Answer 3

You can see this code below as an example if it match your requirement:
ErrorDocument 404 http://example.com/404/
RewriteCond %{REQUEST_URI} ^/404/$
RewriteRule ^(.*)$ /404.php [L]