Uncaught Error: Function name must be a string in /var/www/html/app/code/core/Mage/Core/Model/Layout.php

This error was easy to fix because the problem was in the following line:

$out .= $this->getBlock($callback[0])->$callback[1]();

Instead it should be:

$out .= $this->getBlock($callback[0])->{$callback[1]}();

Source: https://stackoverflow.com/questions/35611696/fatal-error-uncaught-error-function-name-must-be-a-string-in-c-xampp-htdocs-e

Open app/code/core/Mage/Core/Model/Layout.php and inspect the callback invocation shown in the issue. Verify the PHP error and the proposed invocation behavior, then run the relevant Magento request or test to confirm that the layout renders without the function-name error.

Written by the indexing model from the issue text.