Log in a WordPress user programmatically

log_wp_user_programmatically

/*
* Taken from http://www.wprecipes.com/log-in-a-wordpress-user-programmatically
*/
<?php
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 );
    }     
}
?>