Strange behavior from apache2 configuration

I have a really strange situation with apache2 and the way it is running a php
file in some circumstances and offering it up to the browser as text in
others. *The file in question is index.php as the key file using the drupal
content management system.

I have authentication set on most directories in my server configuration so
that if I access the web server from outside it requires authentication. *If
it is from a machine running on the local network it doesn't. *The
configuration is thus

Alias /drupal /usr/share/drupal

<Directory /usr/share/drupal/>
* * * * Options +FollowSymLinks
* * * * AllowOverride All
* * * * #
* * * * # Controls who can get stuff from this server.
* * * * #
* * * * AuthType Basic
* * * * AuthName "Chandler Intranet"
* * * * AuthUserFile /etc/apache2/users
* * * * AuthGroupFile /etc/apache2/groups
* * * * Require group family
* * * * Order allow,deny
* * * * Allow from 192.168.0.0/24
* * * * Satisfy any
</Directory>

The .htaccess file has (amongst other things)

DirectoryIndex index.php

in it, and so in fact does the main server configuration.

When I access the web server with the url http://myserver/drupal from a
192.168.0.0/24 address, it works and index.php gets executed as a php script. *

When I access this url from outside I get prompted for a user name and
password which I enter correctly and then find that the text of index.php not
sent as html (at least I am prompted by Internet Explorer to either save
or open the file. *When I choose Wordpad, I have received the text
inside index.php)

Just to convince myself that php can really run, I have another directory
phptest with an index.php file as follows

<?php phpinfo(); ?>

and when I call this up (http://myserver/phptest ) from outside I get a
detailed web page full of php information.

I just cannot think what it could be about my apache configuration that
changes the behavior dependent on the source of the browser.

Anyone any ideas?

Or ... any ideas about I can understand how exactly the apache configuration
file has been parsed so that I can locate the source of my problem.