I include a php in the my custom firmware, so I need to include a some string into default uhttpd config.
How?
In your buildroot:
$ mkdir -p files/etc/uci-defaults/
$ cat <<EOT > files/etc/uci-defaults/99-uhttpd-set-interpreter
#!/bin/sh
uci add_list uhttpd.main.interpreter='.php=/usr/bin/php-cgi'
uci commit uhttpd
exit 0
EOT
Then rebuild the image with make
. The command above creates a firstboot script that modifies /etc/config/uhttpd before the services are started.
Heh. After some years I'll forget what my actual config is modified in some place.
Alternatively you can simply ship a complete uhttpd config by providing a files/etc/config/uhttpd
in your buildroot.