install php7.3

# install php repository

add-apt-repository ppa:ondrej/php && apt update -y

# install php7.3 and extensions
apt install php7.3 php7.3-fpm php7.3-mbstring php7.3-zip php7.3-gd php7.3-mysql php7.3-curl

# configure php7.3-fpm default pool to run as nginx and listen on specific port instead of unix socket
# open config file with your favourite editor and replace `group www-data` and `user www-data` with `group nginx` and `user nginx` 
# replace `listen /somewhere/php.sock` to `listen 9003`
# vim /etc/php/7.3/fpm/pool.d/www.conf

# restart php7.3-fpm service
systemctl restart php7.3-fpm

# configure nginx to pass cgi to php7.3-fpm
# vim /opt/nginx/site-enabled/xxx.conf replace `fastcgi_pass unix://var/run/php-fpmxxx` to `fastcgi_pass 127.0.0.1:9003`