Example implementation of authenticating users to Invision Powerboard using ssoPublicSessions. Will create a new user if this user is not found. This file is located in admin/sources/classes/session and should be overwritten by you.

ssoPublicSessions.php

<?php
/**
 * Class ssoPublicSessions
 */
class ssoPublicSessions extends publicSessions
{

    protected $ci_user = null;

    public function __construct()
    {
        /* Finish loading publicSessions constructor */
        parent::__construct();

        // bots can be skipped
        if ($this->_member->is_not_human) {
            return;
        }

        if (!$this->ci_user = $this->_loginCheck()) {
            // we do not have a valid session in codeigniter
            self::setMember(0);
            $this->_updateMemberSession();
            return false;
        }

        /** Already logged in, but we had to verify with CI */
        $member = false;
        if (self::$data_store['member_id'] > 0) {

            /** But only if we have our frontend id saved */
            $res = ipsRegistry::DB()->buildAndFetch( array( 'select' => 'field_11 as frontendid',
                'from'   => 'pfields_content',
                'where'  => "member_id = '" . self::$data_store['member_id'] . "'" ) );
            if($res['frontendid'] > 0) {
                // frontend id is set, all is good
                return;
            } else {
                //echo 'saving ;linked id';
                // no frontend id, life is bad, and sad...
                IPSMember::save(self::$data_store['member_id'],array(
                    'customFields' => array(
                        'field_11' => $this->ci_user['userid']
                    )
                ));
                return;
            }

        }

        // we are not logged in, but we would like to!
        $res = ipsRegistry::DB()->buildAndFetch( array( 'select' => 'member_id',
            'from'   => 'pfields_content',
            'where'  => "field_11 = '" . $this->ci_user['userid'] . "'" ) );
        if($res['member_id'] > 0) {
            // echo 'loaded by member id';
            $member = IPSMember::load($res['member_id'], 'all');
        } else {
            // echo 'loaded by username';
            $member = IPSMember::load($this->ci_user['username'], 'all', 'username');
        }

        // IPB didnt find the user
        if (!isset($member['member_id'])) {

            $member = IPSMember::create(array(
                'members' => array(
                    'name' => $this->ci_user['username'],
                    'members_display_name' => $this->ci_user['username'],
                    'password' => rand(111111, 999999),
                    'email' => $this->ci_user['email'],
                ),  /*
        'customFields' => array(
          'frontendid' => $this->ci_user['id']
        )        */
            ), false, true, false);

            IPSMember::save($member['member_id'],array(
                'customFields' => array(
                    'field_11' => $this->ci_user['userid']
                )
            ));

        }

        self::setMember($member['member_id']);
        $this->_updateMemberSession();

        return true;

    }

    /**
     * Check if the user is logged in
     */
    protected function _loginCheck()
    {
        if (!$this->session) $this->session = new FG_Session();
        if ($this->session->userdata('loggedin')) {
            $user = $this->session->all_userdata();
            return $user;
        }
        return false;
    }
}