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>
Edit:
At the time of edit, the v1.3 documentation is down. The v2.4 documentation (current version at time of edit) has a similar stance:
In general, use of .htaccess
files should be avoided when possible. Any configuration that you would consider putting in a .htaccess
file, can just as effectively be made in a <Directory>
section in your main server configuration file.