Explains how to create a secure PHP login script that will allow safe authentication. Features remember-me function using cookies, validates logins on each request to prevent session stealing.

1. How does this work

This is a short explanation why I have chosen these authentication methods.

Users with shell access to the web server can scan valid session id's if the default /tmp directory is used to store the session data.

The protection against this kind of attack is the IP check.

Somebody who has a site (on a shared host with you) can generate valid session for your site.

This is why the checkSession method is used and the session id is recorded in the database.

Somebody may sniff network traffic and catch the cookie.

The IP check should eliminate this problem too.

2. Preparation

You need first to decide what information to store about members, the examples provided will assume almost nothing to make it easier to read.

I will use the PHP 4.1 super global arrays like $_SESSION, $_GET, etc. If you want to make it work on an earlier version of PHP you will have to substitute these with $GLOBALS['HTTP_SESSION_VARS'].

3. Database schema

This is only an example bare structure suitable for online administration, if you want to have registered members you should add more columns.

The schema is somewhat MySQL specific, I have yet to use another database other than MySQL and PostgreSQL but if you are using PostgreSQL you can convert the schema with the example script provided in my article Converting a database schema from MySQL to PostgreSQL.

CREATE TABLE member (
id int NOT NULL auto_increment,
username varchar(20) NOT NULL default '',
password char(32) binary NOT NULL default '',
cookie char(32) binary NOT NULL default '',
session char(32) binary NOT NULL default '',
ip varchar(15) binary NOT NULL default '',
PRIMARY KEY (id),
UNIQUE KEY username (username)
);

The password and cookie fields are md5 hashes which are always 32 octets long. Cookie is the cookie value that is sent to the user if he/she requests to be remembered, session and ip are respectively the session id and the current IP of the visitor.

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