When I calling my script with sudo it would set $USER to root.
$ sudo ./myscript.sh
I tried the chown ${USER:=$(/usr/bin/id -run)}:$USER /var/www/$sitename but it would still return root.
I found if I used who with awk I was able to get the current user that called the script with sudo.
currentuser=$(who | awk '{print $1}')}
chown -R $currentuser:$currentuser /var/www/$sitename`