Skeleton code example for implementing the registration and linking using the functions of Nextend Social Login.

nsl-register-link-skeleton.php

<?php

/**
 * Step1:
 * -Get an access token and put it into an associative array with the key: access_token_data
 * -Read out the providerID from the request.
 */
$providerID                       = 'google';
$access_token                     = '{"access_token":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx","expires_in":3599,"refresh_token":"1\/\/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy","scope":"https:\/\/www.googleapis.com\/auth\/user.birthday.read https:\/\/www.googleapis.com\/auth\/user.phonenumbers.read https:\/\/www.googleapis.com\/auth\/user.addresses.read https:\/\/www.googleapis.com\/auth\/userinfo.email https:\/\/www.googleapis.com\/auth\/userinfo.profile openid","token_type":"Bearer","id_token":"zzzzzzzzzzzzzzzzzzzzzzzzzzzz","created":1664258821}';
$authOptions['access_token_data'] = $access_token;


//Note: The Steam provider works differently, as it is based on OpenID authentication. In case of Steam you need the Claimed ID instead of the access token:
/*
$providerID                       = 'steam';
$openid_claimed_id                = 'https://steamcommunity.com/openid/id/01234567891234567';
$authOptions['openid_claimed_id'] = $openid_claimed_id;
*/


function nslLinkOrRegister($providerID, $authOptions) {
    $provider = NextendSocialLogin::getProviderByProviderID($providerID);
    if ($provider) {
        $social_user_id = $provider->getAuthUserDataByAuthOptions('id', $authOptions);
        if ($social_user_id) {
            /**
             * Step2: Check if the social media account is linked to any WordPress account.
             */
            $wordpress_user_id = $provider->getUserIDByProviderIdentifier($social_user_id);

            if (!is_user_logged_in()) {

                /**
                 * Step3: Handle the logged out users
                 */
                if ($wordpress_user_id !== null) {
                    $provider->triggerSync($wordpress_user_id, $authOptions, "login", true);

                    /**
                     * Step 4: This social media account is already linked to a WordPress account.-> Log the user in using the returned User ID.
                     */

                    return $wordpress_user_id;
                } else {
                    /**
                     * Step 5: This social media account is not linked to any WordPress account, yet. -> Find out if we need to Link or Register
                     */

                    $wordpress_user_id = false;

                    /**
                     * Step 6: Attempt to match a WordPress account with the email address returned by the provider:
                     */
                    $email = $provider->getAuthUserDataByAuthOptions('email', $authOptions);
                    if (empty($email)) {
                        $email = '';
                    } else {
                        $wordpress_user_id = email_exists($email);
                    }

                    if ($wordpress_user_id !== false) {
                        /**
                         * Step 7: There is an email address match -> Link the existing user to the provider
                         */
                        if ($provider->linkUserToProviderIdentifier($wordpress_user_id, $social_user_id)) {
                            $provider->triggerSync($wordpress_user_id, $authOptions, "login", true);

                            //log the user in if the linking was successful

                            return $wordpress_user_id;
                        } else {
                            // Throw error: User already have another social account from this provider linked to the WordPress account that has the email match. They should use that account.
                        }

                    } else {
                        /**
                         * Step 8: There is no email address match -> Register a new WordPress account, e.g. with wp_insert_user()
                         * fill $user_data with the data that the provider returned
                         */
                        $user_data = array(
                            'user_login'   => '',
                            //generate a unique username, e.g. from the name returned by the provider: $provider->getAuthUserDataByAuthOptions('name', $authOptions);
                            'user_email'   => $email,
                            //use the email address returned by the provider, note: it can be empty in certain cases
                            'user_pass'    => '',
                            //generate a password, e.g.: with wp_generate_password()
                            'display_name' => '',
                            //generate a display name, e.g. from the name returned by the provider: $provider->getAuthUserDataByAuthOptions('name', $authOptions);
                            'first_name'   => '',
                            //generate a first name, e.g.: from the first name returned by the provider: $provider->getAuthUserDataByAuthOptions('first_name', $authOptions);
                            'last_name'    => '',
                            //generate a last name, e.g.: from the last name returned by the provider: $provider->getAuthUserDataByAuthOptions('last_name', $authOptions);
                        );


                        $wordpress_user_id = wp_insert_user($user_data);

                        if (!is_wp_error($wordpress_user_id) && $wordpress_user_id) {
                            /**
                             * Step 9: Link the new user to the provider
                             */
                            if ($provider->linkUserToProviderIdentifier($wordpress_user_id, $social_user_id, true)) {
                                $provider->triggerSync($wordpress_user_id, $authOptions, 'register', false);
                                $provider->triggerSync($wordpress_user_id, $authOptions, "login", true);

                                //The registration and the linking was successful -> log the user in.

                                return $wordpress_user_id;
                            }
                        } else {
                            //Throw error: There was an error with the registration
                        }
                    }
                }
            } else {
                /**
                 * Step 10: Handle the linking for logged in users
                 */
                $current_user = wp_get_current_user();
                if ($wordpress_user_id === null) {
                    // Let's connect the account to the current user!
                    if ($provider->linkUserToProviderIdentifier($current_user->ID, $social_user_id)) {
                        //account is linked, we don't need to trigger additional actions we just need to sync the avatar
                        $provider->triggerSync($current_user->ID, $authOptions, false, true);

                        return $current_user->ID;
                    } else {
                        //Throw error: Another social media account is already linked to the current WordPress account. The user need to unlink the currently linked one and he/she can link the this social media account.
                    }
                } else if ($current_user->ID != $wordpress_user_id) {
                    //Throw error: This social account is already linked to another WordPress user.
                }
            }
        }
    }

    return false;
}

try {
    $user_id = nslLinkOrRegister($providerID, $authOptions);
    /**
     * $user_id contains the User ID if the registration/linking was successful or if the account was already linked to an existing account
     * otherwise it returns false.
     * If the user is not logged in already, you can log the user with the $user_id in.
     */
} catch (Exception $e) {
    //handle the exceptions
    return new WP_Error('error', $e->getMessage());
}