<Directory /var/www>
    DirectoryIndex index.htm index.html
</Directory>

<Directory /var/www>
    DirectoryIndex index.php index.htm index.html
</Directory>

% /usr/sbin/apache2 -X &
[1] 16367
(hit page a few times to warm up caches)
% strace -p 16367 -o sys1.txt
Process 16367 attached - interrupt to quit
(hit page once)
Process 16367 detached
% grep stat sys1.txt | grep -v fstat | wc -l
153

stat64("/var/www/index.html", 0xbfd279ac) = -1 ENOENT (No such file or directory)
stat64("/var/www/index.cgi", 0xbfd27afc) = -1 ENOENT (No such file or directory)
stat64("/var/www/index.pl", 0xbfd27afc) = -1 ENOENT (No such file or directory)
stat64("/var/www/index.php", {st_mode=S_IFREG|0664, st_size=7198, ...}) = 0