Install php5.6 curl Module Debian Buster

apt install apt-transport-https lsb-release ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list
apt update
apt install php5.6-curl

I need the curl for PhP5.6 to be used as default curl instead of PhP7.

You should add the PHP5.6 to update-alternative , it will be used later to switch between the installed php version.

let’s say the php5.6 is configured to be installed under /usr/bin/php5.6 , the configuration commands should be:

Adding PHP5.6 to update-alternatives:

update-alternatives --install /usr/bin/php php /usr/bin/php5.6 90

Set PHP5.6 as default:

update-alternatives --set php /usr/bin/php5.6

or you can use:

update-alternatives --config php

Then select php5.6.

The best way is to install the php5.6 and its dependency from Ondřej Surý repository , when a php5.6 update is available , it will be installed through apt.