Download and install https://github.com/pantheon-systems/wp-native-php-sessions on your Pantheon Wordpress site.

add-php-sessions-plugin.php

#!/usr/bin/env php
<?php
/**
 * This script will download and install https://github.com/pantheon-systems/wp-native-php-sessions
 * on your Pantheon Wordpress site.
 * 
 * Works on Macintosh / Linux.
 * 
 * Usage:
 *
 *   terminus auth login
 *   php add-php-sessions-plugin.php my-site-name [env]
 *
 */
$self=array_shift($argv);
$sftp_port="2222";
$site="";
$env="dev";
$pantheon_filename="v0.5.0";
$pantheon_sessions_version="0.5.0";
while (count($argv) > 0) {
  $arg = array_shift($argv);
  if ($arg[0] == '-') {
    switch ($arg) {
      case "--port":
        $sftp_port = array_shift($argv);
        break;
      case "--pantheon-sessions-version":
        $pantheon_sessions_version = array_shift($argv);
        break;
    }
  }
  else {
    if ($site == "") {
      $site = $arg;
    }
    else {
      // Generally, you should only use 'dev' or a multi-dev branch here.
      $env = $arg;
    }
  }
}
function tempdir($dir=FALSE, $prefix='') {
    $tempfile=tempnam($dir ? $dir : sys_get_temp_dir(), $prefix);
    if (file_exists($tempfile)) { unlink($tempfile); }
    mkdir($tempfile);
    if (is_dir($tempfile)) { return $tempfile; }
}
if (empty($site)) {
  print "Usage: $self SITE [ENV]";
  exit(1);
}
// Get the site UUID
$uuid = exec("terminus site info --site=$site --field=id");
// Write the wp-native-php-sessions code to the site with
// the sftp command line tool, and commit it with terminus.
$sftp_host="[email protected].$uuid.drush.in";
// Make sure the site is awake
exec("terminus site wake --site=$site --env=dev");
$mu_plugins_path = 'wp-content/mu-plugins';
$work_dir = tempdir();
chdir($work_dir);
//
// We are going to write the pantheon sessions loader into wp-content/mu-plugins
//
$pantheon_sessions_loader = "<?php
require_once(__DIR__ . '/wp-native-php-sessions/pantheon-sessions.php');";
file_put_contents('00-pantheon-sessions.php', $pantheon_sessions_loader);
//
// Download the pantheon-sessions plugin; we are going to write it to wp-content/mu-plugins as well
//
exec("curl -OL https://github.com/pantheon-systems/wp-native-php-sessions/archive/$pantheon_filename.zip");
rename("$pantheon_filename.zip", "wp-native-php-sessions.zip");
exec('unzip wp-native-php-sessions.zip');
rename("wp-native-php-sessions-$pantheon_sessions_version", 'wp-native-php-sessions');
//
// Make a batch file for the sftp tool
//
$sftp_batch_contents = "cd code/wp-content/mu-plugins
put 00-pantheon-sessions.php
put -r wp-native-php-sessions";
$sftp_batch_file="pantheon-sessions-upload.batch";
file_put_contents($sftp_batch_file, $sftp_batch_contents);
// We need to be in sftp mode to write the file
passthru("terminus site set-connection-mode --site=\"$site\" --env=\"$env\" --mode=sftp");
// Use the sftp cli tool to upload 00-pantheon-sessions.php and wp-native-php-sessions
passthru("sftp -o Port=\"$sftp_port\" -b \"$sftp_batch_file\" \"$sftp_host\"");
// Commit it to the database.
passthru("terminus site code commit --site=\"$site\" --env=\"$env\" --message=\"Add wp-native-php-sessions/pantheon-sessions to the must-use plugins directory.\" --yes");

You are correct -- I updated the gist. I have not tried the whole script in context recently, though.

Hi Greg

I forked and made some changes in order to switch wget to curl (installed pr default for mac users) and to reflect the latest version of the plugin.

Could you review here:
https://gist.github.com/tinefin/f56efd06b8c1d34702363347b4c3fe8f

Diff:
https://gist.github.com/tinefin/f56efd06b8c1d34702363347b4c3fe8f/revisions