Automatic Logins for WordPress

auto-login.php

<?php
class DBrains_Auto_Login {

	protected static $_instance = null;

	function __construct() {
		global $wpdb;

		$this->expires = DAY_IN_SECONDS * 30 * 4;
		$this->table = $wpdb->prefix . 'dbrns_auto_login_keys';

		add_action( 'init', array( $this, 'handle_auto_login' ), 10 );
	}

	public function handle_auto_login() {
		if ( !isset( $_GET['login_key'] ) || !isset( $_GET['user_id'] ) ) {
			return;
		}

		// Limit Login Attempts plugin
		if ( function_exists( 'is_limit_login_ok' ) ) {
			if ( !is_limit_login_ok() ) {
				return;
			}
		}

		$user = new WP_User( $_GET['user_id'] );

		if ( !$user->ID ) {
			return;
		}

		$user_id = $this->get_user_id_for_key( $_GET['login_key'] );

		if ( !$user_id || $user_id != $user->ID ) {
			do_action( 'wp_login_failed', $user->user_login );
			return;
		}

		wp_set_auth_cookie( $user->ID );
		do_action( 'wp_login', $user->user_login, $user );

		$redirect = remove_query_arg( array( 'login_key', 'user_id' ) );
		wp_redirect( $redirect );
		exit;
	}

	public static function instance() {
		if ( is_null( self::$_instance ) ) {
			self::$_instance = new self();
		}
		return self::$_instance;
	}

	public function get_user_id_for_key( $key ) {
		global $wpdb;

		$sql = $wpdb->prepare( 
			"SELECT `user_id`, `created` 
			FROM {$this->table} WHERE `login_key` = %s",
			$key
		);

		$row = $wpdb->get_row( $sql );

		if ( !$row ) {
			return false;
		}

		if ( mysql2date( 'G', $row->created ) < time() - $this->expires ) {
			return false;
		}

		return $row->user_id;
	}

	public function create_key( $user_id ) {
		global $wpdb;

		// Cleanup expired keys
		$expired_date = gmdate( 'Y-m-d H:i:s', ( time() - $this->expires ) );
		$wpdb->query( $wpdb->prepare( "DELETE FROM {$this->table} WHERE `created` < %s", $expired_date ) );

		do {
			$key = wp_generate_password( 40, false );

			$sql = $wpdb->prepare( "SELECT `user_id` FROM {$this->table} WHERE `login_key` = %s", $key );
			$already_exists = $wpdb->get_var( $sql );
		}
		while ( $already_exists );

		$result = $wpdb->insert( $this->table, array(
			'login_key' => $key,
			'user_id' => $user_id,
			'created' => gmdate( 'Y-m-d H:i:s' )
		) );

		if ( !$result ) {
			return false;
		}

		return $key;
	}

}

DBrains_Auto_Login::instance();

auto-login.sql

CREATE TABLE `oiz6q8a_dbrns_auto_login_keys` (
  `login_key` varchar(60) NOT NULL,
  `user_id` bigint(20) NOT NULL,
  `created` datetime NOT NULL,
  PRIMARY KEY (`login_key`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;