suvo@Suvodip-Lenovo ~ $ sudo apt install php-mysqli
[sudo] password for suvo:
Reading package lists... Done`enter code here`
Building dependency tree
Reading state information... Done
Package php-mysqli is a virtual package provided by:
php7.3-mysql 7.3.2-3+ubuntu16.04.1+deb.sury.org+1
php7.2-mysql 7.2.15-1+ubuntu16.04.1+deb.sury.org+1
php7.1-mysql 7.1.26-1+ubuntu16.04.1+deb.sury.org+1
php5.6-mysql 5.6.40-1+ubuntu16.04.1+deb.sury.org+1
php7.0-mysql 7.0.33-1+ubuntu16.04.1+deb.sury.org+1
You should explicitly select one to install.
E: Package `php-mysqli` has no installation candidate
Understand the error message
Package php-mysqli is a virtual package provided by:
[...]
The apt package management uses virtual packages to fulfill dependencies. A virtual package itself has no content at all. It cannot be installed directly. It is meant as an indicator that the expected software was installed to signal other packages that their dependencies are resolved.
apt tells you a list of real packages which install the virtual one. Just install the most recent one (or, if you alread have installed another PHP version, the matching version):
sudo apt install php7.3-mysql