The latest git version of phpseclib should support this. eg.
<?php
include('Net/SSH2.php');
$ssh = new Net_SSH2('127.0.0.1');
$ssh->login('username', 'password');
$ssh->setTimeout(5);
$ssh->startSubsystem('vim');
echo $ssh->read();
This vim subsystem is defined by doing Subsystem vim /usr/bin/vim
in sshd_config.