Description:
------------
Function name as an array element not working even though it's a string

Test script:
---------------

if(false){
//In Magento this doesn't work:-
$out .= $this->getBlock($callback[0])->$callback[1]();

//While this does
$str = $callback[1];
$out .= $this->getBlock($callback[0])->$str();
}

Expected result:
----------------
function is to be called

Actual result:
--------------
Throws exception:-


[Sun Dec 20 10:09:53.861694 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: PHP message: PHP Fatal error:  Uncaught Error: Function name must be a string in /var/www/www.popcherry.com.au/pcmago14/source/app/code/core/Mage/Core/Model/Layout.php:555
[Sun Dec 20 10:09:53.861764 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: Stack trace:
[Sun Dec 20 10:09:53.861777 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: #0 /var/www/www.popcherry.com.au/pcmago14/source/app/code/core/Mage/Core/Controller/Varien/Action.php(390): Mage_Core_Model_Layout->getOutput()
[Sun Dec 20 10:09:53.861786 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: #1 /var/www/www.popcherry.com.au/pcmago14/source/app/code/core/Mage/Cms/Helper/Page.php(137): Mage_Core_Controller_Varien_Action->renderLayout()
[Sun Dec 20 10:09:53.861795 2015] [:error] [pid 3622:tid 140406412060416] 
[client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: #2 /var/www/www.popcherry.com.au/pcmago14/source/app/code/core/Mage/Cms/Helper/Page.php(52): Mage_Cms_Helper_Page->_renderPage(Object(Mage_Cms_IndexController), 'home')
[Sun Dec 20 10:09:53.861804 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: #3 /var/www/www.popcherry.com.au/pcmago14/source/app/code/core/Mage/Cms/controllers/IndexController.php(45): Mage_Cms_Helper_Page->renderPage(Object(Mage_Cms_IndexController), 'home')
[Sun Dec 20 10:09:53.861812 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: #4 /var/www/www.popcherry.com.au/pcmago14/source/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Cms_IndexController->indexAction()
[Sun Dec 20 10:09:53.861821 2015] [:error] [pid 3622:tid 140406412060416] [client 110.174.8.215:55270] FastCGI: server "/opt/php-7.0.1/sbin/php-fpm" stderr: #5 /var/www/www.popcherry.com.au/pcmago14/sou...