first commit - seems to work

sntg-automatic-user-deletion.php

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+<?php
+/* 
+ *	Plugin Name: Sonith's Automatic User Deletion 
+ *  Author: CRP92
+ *  Description: The Plugin autmatically deletes users (except admins) that have not been logged in for more than one year and assigns the content to a default user (sntg-deleted-user)
+ *
+ * License: GPLv3 or later
+ * License URI: https://www.gnu.org/licenses/gpl-3.0.en.html
+ * Version: 0
+ * Text Domain: sntg-automatic-user-deletion
+ */
+
+include ('src/constants.php');
+include ('src/login_tracking.php');
+include ('src/cron_job.php');
+
+/********** HELPER FUNCTIONS *********************/
+if ( ! function_exists('write_log')) {
+	/** function for easily writing data to the log */
+	function write_log ( $log )  {
+       if ( is_array( $log ) || is_object( $log ) ) {
+          error_log( print_r( $log, true ) );
+       } else {
+          error_log( $log );
+       }
+    }
+ }
+/********** END HELPER FUNCTIONS *****************/
+
+register_activation_hook( __FILE__, 'sntg_automatic_user_deletion_activation' );
+register_deactivation_hook( __FILE__, 'sntg_automatic_user_deletion_deactivation' );
+
+add_action('admin_init', 'sntg_automatic_user_deletion_activate');
+add_action('wp_login', 'sntg_aud_login_tracking');
+add_action('sntg_automatic_user_deletion_cron', 'sntg_aud_cron_job');
+
+function sntg_aud_activation(){
+    add_option( 'sntg_activated_plugin', 'sntg-automatic-user-deletion' );
+}
+
+/** registers new user with DELETED_USER_NAME "sntg-deleted-user" that stores all old data
+ *  registers cron job ("sntg-check-inactive-users" - once a week)
+ *  adds action to wp-login to set last login
+ */
+function sntg_automatic_user_deletion_activate(){
+
+    if (is_admin() && get_option ('sntg_activated_plugin') == 'sntg-automatic-user-deletion'){
+        //create new user for orphaned content
+        if (!username_exists(DELETED_USER_NAME)){
+            wp_create_user(DELETED_USER_NAME, wp_generate_password());
+        }
+        //schedule the checking function
+        if (!wp_next_scheduled ('sntg_automatic_user_deletion_cron')){
+            wp_schedule_event(time(), SCHEDULING_INTERVAL, 'sntg_automatic_user_deletion_cron');
+        }
+    }
+    if (username_exists(DELETED_USER_NAME) && wp_next_scheduled ('sntg_automatic_user_deletion_cron')){
+        delete_option( 'sntg_activated_plugin' );
+    }
+}
+
+function sntg_automatic_user_deletion_deactivation(){
+    write_log('hm...');
+    $timestamp = wp_next_scheduled( 'sntg_automatic_user_deletion_cron' );
+    wp_unschedule_event( $timestamp, 'sntg_automatic_user_deletion_cron', array() );
+    delete_option( 'sntg_deactivated_plugin' );
+}

src/constants.php

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+<?php
+define ('SEC_TO_MONTH', 24*60*60);
+define ('USER_DELETION_DELAY', 12); //how many months does plugin wait before users are deleted?
+define ('DELETION_DELETION_DELAY',36); //how many months does the plugin wait before user deletion information is deleted
+define ('DELETED_USER_NAME', 'sntg-deleted-user'); //name of user that "collects" deleted users content (posts, ...)
+define ('SCHEDULING_INTERVAL', 'weekly'); //how often should the cron job run?
+define ('SUBJECT', 'Baldige Löschung Ihres Benutzerkontos'); //subject of the warning email the user gets

src/cron_job.php

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+<?php 
+
+/** cron hooking function 
+ * deletes users that were prepared last week
+ * prepares users for deletion next week
+ * deletes deletion info that is older than DELETION_DELETION_DELAY (months)
+*/
+function sntg_aud_cron_job(){
+    delete_prepared_users();
+    prepare_users_for_deletion();
+    delete_deletion_entries();
+}
+
+/** delete users that were prepared the last time the job ran */
+function delete_prepared_users(){
+    $users = get_users( array( 'fields' => array( 'ID' ) ) );
+    foreach($users as $user){
+        $usermeta = get_user_meta ( $user->ID, 'sntg_user_to_be_deleted');
+        if (sizeof($usermeta) == 1){
+            write_log("user " . $user->ID . " has to be deleted: " .$usermeta[0]);
+            delete_user($user->ID);
+        }
+        else {
+            echo var_dump($usermeta);
+        }
+    }
+}
+
+/** add db entries for non-admin users (wp_user_level < 10) 
+ * that have not been logged in for longer than USER_DELETION_DELAY (->constants.php)
+ * 
+ */
+function prepare_users_for_deletion(){
+    //write_log("prepare users for deletion");
+    $users = get_users( array( 'fields' => array( 'ID' ) ) );
+    foreach($users as $user){
+        $wp_user_level = get_user_meta ( $user->ID, 'wp_user_level', true); 
+        $user_object = get_user_by('id', $user->ID);
+        //only delete non admins
+        if (($wp_user_level < 10)  && ($user_object->user_login != DELETED_USER_NAME )){
+            $last_login = get_user_meta ($user->ID, 'sntg_last_login', true);
+            //if no entry exists, create one with date now
+            if ($last_login == ''){
+                update_user_meta($user->ID, 'sntg_last_login', time(), true);
+            }
+            //otherwise check if it was more then USER_DELETION_DELAY (one year) ago
+            elseif ( ( (time() - $last_login) / SEC_TO_MONTH) > USER_DELETION_DELAY){
+                    update_user_meta($user->ID, 'sntg_user_to_be_deleted', time(), true);
+                    write_log("deletion mail sent: " . send_deletion_warning_mail($user->ID));
+                    write_log($user_object->user_login . " prepared for deletion");
+            }
+        }
+    }
+}
+
+/** creates an db entry with info about deletion, reassigns users content to default user (DELETED_USER_NAME -> constants.php) and deletes the user */
+function delete_user($old_user_ID){
+    $reassign_user = get_user_by('login', DELETED_USER_NAME);
+    $reassign_user_id = $reassign_user->ID;
+    write_log ("deleting user with id: " . $old_user_ID);
+    create_deletion_entry($old_user_ID);
+    require_once(ABSPATH.'wp-admin/includes/user.php');
+    wp_delete_user($old_user_ID, $reassign_user_id);
+}
+
+/** creates a post with info about deleted user */
+function create_deletion_entry($user_id){
+    $user = get_user_by('id', $user_id);
+    $info =     "user id: " . $user_id . "; " .
+                "email: " . $user->user_email . '; ' . 
+                "username: " . $user->user_login .";";
+    wp_insert_post(array(
+            'post_type' => 'sntg_ud_info',
+            'post_content' => $info,
+            'post_status' => 'publish'
+    ));
+}
+
+/** sends an email (templates/info_mail.php) to user with subject SUBJECT (->constants.php) */
+function send_deletion_warning_mail($user_id){
+    $user = get_user_by('id', $user_id);
+    add_filter( 'wp_mail_content_type', function( $content_type ) {
+        return 'text/html';
+    });
+    $subject = get_bloginfo ('name') . ': ' . SUBJECT;  //set subject in constants.php
+
+    ob_start();
+    include (ABSPATH . 'wp-content/plugins/sntg-automatic-user-deletion/templates/info_mail.php');
+    $message = ob_get_clean();
+
+    wp_mail($user->user_email, $subject, $message);
+}
+
+/** after DELETION_DELETION_DELAY, most of the info about deleted users (eg email address) 
+ * has to be deleted as well (DSGVO), but the info that a user was deleted has to be stored*/
+function delete_deletion_entries(){
+    $delay_years = intdiv( DELETION_DELETION_DELAY, 12 );
+    $delay_months = DELETION_DELETION_DELAY % 12;
+    $entries_to_delete = get_posts(array(
+        'post_type'     => 'sntg_ud_info',
+        'numberposts'   => '-1',
+        'date_query' => array(
+            array(
+                'before' => array(
+                    'year'  => date('Y') - $delay_years,
+                    'month' => date('n') - $delay_months,
+                ),
+                'inclusive' => true,
+            )
+        ),
+    ));
+    foreach ($entries_to_delete as $entry){
+        $info = "deleted deletion info. post_id: " . $entry->ID . "; " . explode(";", $entry->post_content)[0];
+        write_log($info);
+        wp_insert_post(array(
+            'post_type' => 'sntg_ud_deletion',
+            'post_content' => $info,
+            'post_status' => 'publish'
+        ));
+        wp_delete_post ($entry->ID);
+    }
+}

src/login_tracking.php

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+<?php
+
+/** login tracking function */
+function sntg_aud_login_tracking(){
+    $user_id = get_current_user_id();
+    //update last login time
+    write_log('sntg_automatic_user_deletion_login_tracking');
+    update_user_meta($user_id, 'sntg_last_login', time());
+    //remove deletion entry if it exists
+    delete_user_meta($user_id, 'sntg_user_to_be_deleted');
+}

templates/info_mail.php

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+<?php
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+?>
+
+<h1>Hallo <?php echo $user->login ?></h1>
+
+<p>Du hast dein Nutzerkonto auf <a href="<?php echo get_bloginfo('url') ?> "> <?php echo get_bloginfo ('name')?> </a> seit ca. einem Jahr nicht mehr genutzt. Aus Datenschutzgründen wird es daher in einer Woche gelöscht, falls du dich bis dahin nicht einloggst. <p>
+
+<p>Liebe Grüße, <br> <?php echo get_bloginfo ('name')?>  - <?php echo get_bloginfo ('description')?>