// File: includes.php

<?php

    // Login Script

    if(isset($_POST['login'])){
        // Grab the username based on the form entry...

        $usr_name = mysql_real_escape_string(addslashes($_POST['usr_name']));
        $usr_pass = mysql_real_escape_string(addslashes($_POST['usr_pass']));

        // Compare entries based on SQL or RAW variables.
        // For the sake of this example, we will use RAW variables... but you will want to use an SQL table for dynamic users.

        $username = 'Foo';
        $password = 'Bar';
        $userid = '1'; // Would be grabbed from a table.

        if($usr_name != $username){
            echo 'There was a problem with the username or password you entered.  Please go back and try again.';
        }
        elseif($usr_password != $password){
            echo 'There was a problem with the username or password you entered. Please go back and try again.';
        }
        else{
            // Preform login...

            setcookie('user',$userid,time()+3600);
            echo 'Congratulations, your login was successful.';
        }
    }

    // End Login Script


?>
// FIle: logout.php

<?php
    // Logout Script...
    if(isset($_GET['usr']){
        setcookie('user',$_GET['user'],time()-5200);
    }
    else{
        echo 'No valid cookie id... you fail at php nub.';
    }
?>


// File: index.php

<php

require('includes.php');

    // Check to see if user is logged in...

    if(isset($_COOKIE['user'])){
        // Extra check for security... Compare with the DB / Raw variable.
        // Normally you would do an SQL statement to see if the Id exists, but in our example we will just draw the raw variable from the includes.php.

        $realid = '1'; // Again would be drawn from a DB query.

        if($realid != $userid){
            echo 'Your cookie was falsely set, please <a href="logout.php?usr=' . $userid . '">Logout</a> and attempt to login again.';
        }
        else{
            echo 'Main page, logged in... hello world!';

            //Here is where you would put your main page's content... whatever that might be.
        }
    }
    else{
        //Not logged in, ergo print the login form.

        echo '<form action="" method="post">',
            'Username: <input type="text" name="usr_name" /><br />',
            'Password: <input type="password" name="usr_password" /><br />',
            '<input type="submit" name="login" value="Process Login" />';
    }

?>