Installation of php-ssh2 on DirectAdmin is very simple. Here is how I install it, please note that I use DirectAdmin 1.51.4, CentOS 7x64, PHP 7.0 FPM on Vultr server.
Pre-install requirements
On CentOS, php-ssh2 requires libssh2-devel, so we install it
yum install libssh2-devel
Install PHP SSH2
As of php 7, we need latest ssh2 package, let download it
cd /usr/local/src/
wget https://pecl.php.net/get/ssh2-1.1.tgz
tar -xzvf ssh2-1.1.tgz
cd ssh2-1.1
Build the extension
/usr/local/php70/bin/phpize
./configure --with-php-config=/usr/local/php70/bin/php-config
make && make install
Update php.ini
just add extension=ssh2.so
to your php.ini
file, then restart Apache/PHP-FPM
That's it!