While adding 'DirectoryIndex index.php' to a .htaccess file may work,
NOTE:
In general, you should never use .htaccess files
This is quoted from http://httpd.apache.org/docs/1.3/howto/htaccess.html
Although this refers to an older version of apache, I believe the principle still applies.
Adding the following to your httpd.conf (if you have access to it) is considered better form, causes less server overhead and has the exact same effect:
<Directory /myapp>
DirectoryIndex index.php
</Directory>