function custom_login() {
    $creds = array();
    $creds['user_login'] = 'example';
    $creds['user_password'] = 'plaintextpw';
    $creds['remember'] = true;
    $user = wp_signon( $creds, false );
    if ( is_wp_error($user) )
        echo $user->get_error_message();
}
// run it before the headers and cookies are sent
add_action( 'after_setup_theme', 'custom_login' );

$user_id = username_exists( $user_name );
if ( !$user_id and email_exists($user_email) == false ) {
    $random_password = wp_generate_password( $length=12,
    $include_standard_special_chars=false );
    $user_id = wp_create_user( $user_name, $random_password, $user_email );
} else {
    $random_password = __('User already exists.  Password inherited.');
}