Example of PrestaShop module override, place this file in override/modules/blocklink/

blocklink.php

<?php
/*
* 2007-2016 PrestaShop
*
* NOTICE OF LICENSE
*
* This source file is subject to the Open Software License (OSL 3.0)
* that is bundled with this package in the file LICENSE.txt.
* It is also available through the world-wide-web at this URL:
* http://opensource.org/licenses/osl-3.0.php
* If you did not receive a copy of the license and are unable to
* obtain it through the world-wide-web, please send an email
* to [email protected] so we can send you a copy immediately.
*
* DISCLAIMER
*
* Do not edit or add to this file if you wish to upgrade PrestaShop to newer
* versions in the future. If you wish to customize PrestaShop for your
* needs please refer to http://www.prestashop.com for more information.
*
*  @author PrestaShop SA <[email protected]>
*  @copyright  2007-2016 PrestaShop SA
*  @version    Release: $Revision$
*  @license    http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
*  International Registered Trademark & Property of PrestaShop SA
*/
class BlockLinkOverride extends BlockLink {
	public function install()
    {
        parent::install();
       	return $this->registerHook('displayFooter');
    }
	public function hookDisplayFooter($params)
	{
		return $this->hookLeftColumn($params);
	}
}