Fatal error: Uncaught Error: Function name must be a string app\code\core\Mage\Core\Model\Layout.php:555
It happens because in PHP 7 you need to clarify that you are going to call the $callback variable as a method (function). So, the original line of the code looks like the following (file app/code/core/Mage/Core/Model/Layout.php
):
$out .= $this->getBlock($callback[0])->$callback[1]();
In order to make it work on the latest PHP version we need to replace this piece of code by this one:
$out .= $this->getBlock($callback[0])->{$callback[1]}();