Plugin
register_activation_hook() ----------Registers the function to activate the plugin.
register_deactivation_hook() ----------Registers the function to deactivate the plugin.
register_uninstall_hook() ----------Registers the function for deleting the plugin.
plugins_url() ----------Gets URL of plugins/mu plugins folder (no slash at the end)**.
plugin_basename() ----------Path to plugin file (from plugins folder).
plugin_dir_path() ----------Gets Path to plugin folder (has slash at the end).
plugin_dir_url() ----------Gets URL of the plugin folder (has slash at the end).
get_plugins() ----------Gets data of plugins (active and inactive).
get_plugin_data() ----------Gets data of the plugin from file headers.
activate_plugins() ----------Activates plugins.
deactivate_plugins() ----------Deactivates plugins.
is_plugin_active() ----------Checks if a plugin is active. Works only in the admin panel.
uninstall.php
<?php
if( ! defined('WP_UNINSTALL_PLUGIN') ) exit;
// the check passed successfully. Starting from here remove all plugin data (options, db tables etc.).
delete_option( 'plug_option' );
