$ sudo apt-get update
$ sudo apt-get install libssh2-php 

libssh2-php is installed in your system.

Make ensure the libssh2-php package were installed using the commands given below,

$ sudo dpkg-query -l | grep libssh2-php *

You will get with libssh2-php package name, version, architecture and description in a table

libssh2-php package basic information: