Class 'session' not found in Webserver with Php 7.0

I am deploying an application developed in PHP on a server Linux Ubuntu 16.04 with PHP 7.0 and Nginx .

At the time of authentication in the application, the following line is presented in the Nginx error log:

  

2016/11/08 22:53:30 [error] 26138 # 26138: * 58 FastCGI sent in stderr: "PHP message: PHP Fatal error: Class 'session' not found in / data / xxxxxx / xxxxxx / httpdocs /html/validarusuario.php on line 4 "while reading upstream, client: 10.0.0.59, server: localhost, request:" POST / validateuser.php HTTP / 1.1 ", upstream:" fastcgi: // unix: / var / run /php-fpm-appmanagement.sock: ", host:" 10.0.1.37 ", referrer:" link "

The line that the error refers to is the following:

<?php
    // Arma la sesi  n
    include "sesion.class.php";
    $session = new session();
    $session->inicio_sesion('_s', false);

What is happening?