DirIndex

To control what files are searched for in what order in a directory, edit /etc/apache2/mods-available/dir.conf and modify the line

DirectoryIndex index.html index.cgi index.pl index.php index.xhtml index.htm

to e.g.

DirectoryIndex index.php index.html index.cgi index.pl index.xhtml index.htm

if you want index.php to take preference over index.html. Apache searches for the first file in this list that it finds, and serves that.