The current PHP ssh2 extension via PECL won't compile under PHP7.
Using a more recent version via PHP's GitHub we can make this work.
$ sudo apt-get install autoconf libssh2-1-dev
$ curl -LO https://github.com/php/pecl-networking-ssh2/archive/master.zip
$ unzip master.zip
$ cd pecl-networking-ssh2-master
$ phpize
$ ./configure
$ make
Extension will be available for use at ./modules/ssh2.so
.
This is really old, just putting a note here for future reference as you shouldn't use the master branch.
You can install the most recent version now, which supports PHP 7, by appending the version number: