Override php module core file

With this code, you can create MODULE_NAME.php in override/modules/MODULE_NAME/ which will override the module core.

As example :
/override/modules/blocknewsletter/blocknewsletter.php :

<?php
class BlockNewsletterOverride extends BlockNewsletter
{
    	protected function register($email, $register_status)
	{
                // do something
		return parent::register($email, $register_status);
	}
}