Uninstall and cleanup hooks in Wordpress

Introduction

Uninstall and cleanup hooks help remove plugin data when you delete the plugin. This keeps your website clean and fast.

You want to delete plugin settings when the plugin is removed.

You want to clear custom database tables created by your plugin.

You want to remove options or user meta added by your plugin.

You want to avoid leaving unused data that slows down your site.

You want to keep your WordPress site tidy after uninstalling plugins.

Syntax

Wordpress

<?php
register_uninstall_hook(__FILE__, 'my_plugin_uninstall');

function my_plugin_uninstall() {
    // Cleanup code here
    delete_option('my_plugin_option');
    // Remove custom tables or meta
}
?>

The register_uninstall_hook function links your cleanup function to plugin uninstall.

The cleanup function should delete all plugin data like options, tables, or metadata.

Examples

This example deletes a normal option and a network-wide option on uninstall.

Wordpress

<?php
register_uninstall_hook(__FILE__, 'cleanup_plugin_data');

function cleanup_plugin_data() {
    delete_option('plugin_setting');
    delete_site_option('plugin_network_setting');
}

This example deletes a custom database table created by the plugin.

Wordpress

<?php
register_uninstall_hook(__FILE__, 'remove_custom_table');

function remove_custom_table() {
    global $wpdb;
    $table = $wpdb->prefix . 'custom_table';
    $wpdb->query("DROP TABLE IF EXISTS $table");
}

This example removes user meta data added by the plugin.

Wordpress

<?php
register_uninstall_hook(__FILE__, 'delete_user_meta_data');

function delete_user_meta_data() {
    global $wpdb;
    $wpdb->query("DELETE FROM $wpdb->usermeta WHERE meta_key = 'plugin_user_meta'");
}

Sample Program

This plugin creates an option and a custom table on activation. When uninstalled, it deletes the option, drops the table, and removes user meta data related to the plugin.

Wordpress

<?php
/**
 * Plugin Name: Cleanup Example
 */

register_uninstall_hook(__FILE__, 'cleanup_example_uninstall');

function cleanup_example_uninstall() {
    // Remove plugin option
    delete_option('cleanup_example_option');

    // Remove custom table
    global $wpdb;
    $table_name = $wpdb->prefix . 'cleanup_example_table';
    $wpdb->query("DROP TABLE IF EXISTS $table_name");

    // Remove user meta
    $wpdb->query("DELETE FROM $wpdb->usermeta WHERE meta_key = 'cleanup_example_user_meta'");
}

// Plugin activation creates option and table for demonstration
register_activation_hook(__FILE__, 'cleanup_example_activate');
function cleanup_example_activate() {
    add_option('cleanup_example_option', 'some value');
    global $wpdb;
    $table_name = $wpdb->prefix . 'cleanup_example_table';
    $charset_collate = $wpdb->get_charset_collate();
    $sql = "CREATE TABLE $table_name (
        id mediumint(9) NOT NULL AUTO_INCREMENT,
        data text NOT NULL,
        PRIMARY KEY  (id)
    ) $charset_collate;";
    require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
    dbDelta($sql);
}

Important Notes

Uninstall hooks only run when the plugin is deleted from the WordPress admin, not when deactivated.

Always test uninstall code on a backup site to avoid accidental data loss.

Use register_uninstall_hook or an uninstall.php file for cleanup.

Summary

Uninstall hooks help remove all plugin data when deleting the plugin.

Use register_uninstall_hook to link a cleanup function.

Cleanup includes deleting options, custom tables, and user meta.

Practice

1. What is the main purpose of using register_uninstall_hook in a WordPress plugin?

easy

A. To register a new shortcode for the plugin

B. To add a settings page in the admin dashboard

C. To clean up plugin data when the plugin is deleted

D. To enqueue scripts and styles for the plugin

Solution

1. Step 1: Understand the role of uninstall hooks

   Uninstall hooks are designed to remove all plugin data when the plugin is deleted to keep the site clean.

2. Step 2: Identify the function of register_uninstall_hook

   This function links a cleanup function that runs on plugin deletion to remove options, transients, and custom tables.

3. Final Answer:

   To clean up plugin data when the plugin is deleted -> Option C

4. Quick Check:

   Uninstall hook purpose = clean up data [OK]

Hint: Uninstall hooks remove plugin data on deletion [OK]

Common Mistakes:

• Confusing uninstall hooks with activation hooks
• Thinking uninstall hooks add features instead of cleaning
• Using uninstall hook to enqueue scripts

2. Which of the following is the correct way to register an uninstall hook in a WordPress plugin?

easy

A. register_activation_hook(__FILE__, 'my_cleanup_function');

B. add_action('uninstall_plugin', 'my_cleanup_function');

C. add_filter('uninstall_hook', 'my_cleanup_function');

D. register_uninstall_hook(__FILE__, 'my_cleanup_function');

Solution

1. Step 1: Recall the correct function for uninstall hooks

   The correct function to register an uninstall hook is register_uninstall_hook with the plugin file and cleanup function.

2. Step 2: Check the syntax of each option

   register_uninstall_hook(__FILE__, 'my_cleanup_function'); uses the correct function and parameters. Options A, B, and D use incorrect functions or hooks.

3. Final Answer:

   register_uninstall_hook(__FILE__, 'my_cleanup_function'); -> Option D

4. Quick Check:

   Correct uninstall hook syntax = register_uninstall_hook(__FILE__, 'my_cleanup_function'); [OK]

Hint: Use register_uninstall_hook with __FILE__ and function name [OK]

Common Mistakes:

• Using activation hook instead of uninstall hook
• Using add_action with wrong hook name
• Confusing filters with hooks

3. Consider this uninstall function in a plugin:

function cleanup_plugin() {
  delete_option('plugin_setting');
  delete_transient('plugin_cache');
  global $wpdb;
  $wpdb->query("DROP TABLE IF EXISTS {$wpdb->prefix}plugin_data");
}

What will happen when the plugin is uninstalled?

medium

A. All plugin data including options, transients, and custom tables will be removed

B. Only the option 'plugin_setting' will be deleted

C. The transient 'plugin_cache' will be deleted but the table remains

D. Nothing will be deleted because uninstall hooks do not run functions

Solution

1. Step 1: Analyze the cleanup function actions

   The function deletes an option, a transient, and drops a custom database table using SQL.

2. Step 2: Understand uninstall hook behavior

   When the uninstall hook calls this function, all these cleanup actions will run, removing all plugin data.

3. Final Answer:

   All plugin data including options, transients, and custom tables will be removed -> Option A

4. Quick Check:

   Cleanup function deletes all data = All plugin data including options, transients, and custom tables will be removed [OK]

Hint: Cleanup function removes all data types explicitly [OK]

Common Mistakes:

• Assuming uninstall hooks do not run cleanup functions
• Thinking only options or transients are deleted
• Ignoring the SQL DROP TABLE command

4. Identify the error in this uninstall hook registration:

register_uninstall_hook('my-plugin/my-plugin.php', 'cleanup_plugin');

function cleanup_plugin() {
  delete_option('my_plugin_option');
}

medium

A. The plugin file path should use __FILE__ instead of a string

B. The cleanup function name must be prefixed with 'uninstall_'

C. delete_option cannot be used in uninstall functions

D. The uninstall hook must be registered inside the cleanup function

Solution

1. Step 1: Check the first parameter of register_uninstall_hook

   The first parameter should be the plugin main file path, usually passed as __FILE__ for reliability.

2. Step 2: Understand why using a string path is problematic

   Using a string path can cause the hook not to trigger if the path is incorrect or changes; __FILE__ ensures correct reference.

3. Final Answer:

   The plugin file path should use __FILE__ instead of a string -> Option A

4. Quick Check:

   Use __FILE__ for plugin path in uninstall hook [OK]

Hint: Always use __FILE__ for plugin path in uninstall hook [OK]

Common Mistakes:

• Using string path instead of __FILE__
• Thinking function name must have uninstall prefix
• Registering hook inside cleanup function

5. You want to ensure your plugin cleans up all its data on uninstall, including options, transients, and a custom table named wp_custom_data. Which of the following uninstall functions correctly performs this cleanup?

hard

A. function uninstall_plugin() {
delete_option('plugin_option');
delete_transient('plugin_transient');
global $wpdb;
$wpdb->query("DROP TABLE wp_custom_data");
}

B. function uninstall_plugin() {
delete_option('plugin_option');
delete_transient('plugin_transient');
global $wpdb;
$wpdb->query("DROP TABLE IF EXISTS wp_custom_data");
}

C. function uninstall_plugin() {
delete_option('plugin_option');
delete_transient('plugin_transient');
global $wpdb;
$wpdb->query("DELETE FROM wp_custom_data");
}

D. function uninstall_plugin() {
delete_option('plugin_option');
delete_transient('plugin_transient');
}

Solution

1. Step 1: Review each cleanup action

   function uninstall_plugin() { delete_option('plugin_option'); delete_transient('plugin_transient'); global $wpdb; $wpdb->query("DROP TABLE IF EXISTS wp_custom_data"); } deletes the option, transient, and drops the custom table safely using IF EXISTS.

2. Step 2: Compare with other options

   function uninstall_plugin() { delete_option('plugin_option'); delete_transient('plugin_transient'); global $wpdb; $wpdb->query("DROP TABLE wp_custom_data"); } drops table without IF EXISTS (may cause error if table missing). function uninstall_plugin() { delete_option('plugin_option'); delete_transient('plugin_transient'); global $wpdb; $wpdb->query("DELETE FROM wp_custom_data"); } deletes rows but not table. function uninstall_plugin() { delete_option('plugin_option'); delete_transient('plugin_transient'); } misses table cleanup.

3. Final Answer:

   function uninstall_plugin() { delete_option('plugin_option'); delete_transient('plugin_transient'); global $wpdb; $wpdb->query("DROP TABLE IF EXISTS wp_custom_data"); } -> Option B

4. Quick Check:

   Safe drop table + delete option and transient = function uninstall_plugin() { delete_option('plugin_option'); delete_transient('plugin_transient'); global $wpdb; $wpdb->query("DROP TABLE IF EXISTS wp_custom_data"); } [OK]

Hint: Use DROP TABLE IF EXISTS to safely remove custom tables [OK]

Common Mistakes:

• Dropping table without IF EXISTS causing errors
• Deleting rows instead of dropping table
• Forgetting to remove custom tables