Install the necessary packages before you can build/install ssh2 extension
yum install gcc php-devel php-pear libssh2 libssh2-devel make
Install the extension, (hit enter for autodetect when it prompts you)
pecl install -f ssh2
Once the install is completed, you just have to tell PHP to load the extension when it boots.
echo extension=ssh2.so > /etc/php.d/ssh2.ini
Restart your webserver and test to see if the changes took effect.
service httpd restart
You can check it installed with the following command
php -m | grep ssh2