If you are experiencing, a similar error to below:
Fatal error: Uncaught Error: Function name must be a string in C:\xampp\htdocs\acf\app\code\core\Mage\Core\Model\Layout.php:555 Stack trace: #0
This happens because in you are running Magneto 1.9 in PHP 7. To solve the error you need to clarify that you are going to call the $callback variable as a method (function).
Navigate to the original line of the code in file:
app/code/core/Mage/Core/Model/Layout.php
Find the line ~line 555
$out .= $this->getBlock($callback[0])->$callback[1]();
And replace it with
$out .= $this->getBlock($callback[0])->{$callback[1]}();