Log in a WordPress user programmatically

functions.php

<?php

/*
* Here is the function, drop it in your functions.php file
*/

function auto_login( $user ) {
    $username = $user;
    if ( !is_user_logged_in() ) {
        $user = get_userdatabylogin( $username );
        $user_id = $user->ID;
        wp_set_current_user( $user_id, $user_login );
        wp_set_auth_cookie( $user_id );
        do_action( 'wp_login', $user_login );
    }     
}

/*
* Then, to log in a user, do the following
* Replace 'admin' with your username.
*/
auto_login( 'admin' );