The behaviour you describe, that is going to
http://example.com/index.php when you visit the URL in
http://example.com/, should be handled by Apache by default.
However there are a variety of default handlers that Apache may use, chosen based on the order they are supplied in the configuration file. For example if you had an index.html and an index.php file, it is likely that Apache will default to the .html file if present, instead of defaulting to the .php file.
The most straightforward solution would be to remove all index.* files, leaving only index.php.
If this is what you have done, you should contact your host to see if Apache is configured correctly.