Hello,
the easiest way is too install php5.6-fpm with :
apt update && apt install php5.6-fpm php5.6-curl php5.6-gd php5.6-imap php5.6-mcrypt php5.6-readline php5.6-common php5.6-recode php5.6-mysql php5.6-cli php5.6-curl php5.6-mbstring php5.6-bcmath php5.6-mysql php5.6-opcache php5.6-zip php5.6-xml php5.6-soap -y
Then to overwrite the file /etc/php/5.6/fpm/pool.d/www.conf with the following content :
[www]
user = www-data
group = www-data
listen = 127.0.0.1:9000
listen.owner = www-data
listen.group = www-data
pm = ondemand
pm.max_children = 100
pm.start_servers = 20
pm.min_spare_servers = 10
pm.max_spare_servers = 30
chdir = /
ping.path = /ping
pm.status_path = /status
pm.max_requests = 500
request_terminate_timeout = 300
Last step is to replace fastcgi_pass php72;
by fastcgi_pass php;
in your vhost.