To support both index.html and index.php same time:
uci set uhttpd.main.index_page='index.php index.html'
uci commit
/etc/init.d/uhttpd restart
uci show uhttpd
...
uhttpd.main.index_page=index.html index.php
...
nano /etc/config/uhttpd
...
option index_page 'index.html index.php'
...
The order of index.html index.php is very important, the last one has high high priority.
"index.html index.php" ; index.php shows first, if index.php is missing then shows index.html.
"index.php index.html"; index.html shows first.