There are two types of redirect: an alias/pointer, or a real redirect.
In the first case, Apache get a request and answers with the correct data as if it were the requested data.
For example, the directive DirectoryIndex in httpd.conf is a list of files Apache will look into (one by one, until one exists) and give as answer to the "/" request. That's why when you open
www.domain.example/ you will receive
www.domain.example/index.php
If that's what you want, check DirectoryIndex.
The second case triggers an HTTP 3xx Redirect code, it's a special answer that tells the client to look for another file. For example RewriteRule will do it by default.
If you want to
see "/index.php" in the client when looking for "/", you will have to insert this in a ".htaccess" file in the same directory:
Code:
RewriteEngine On
RewriteRule ^/$ /index.php [R=301,L]
The 301 code is a permanent redirect, this way most search engines will know "/" doesn't exist anymore and add it's ranking value to "/index.php".