This error happens because the version of Joomla! you are running is not compatible with PHP 5.3.0 . You could get this error for the reason being that your host upgraded to PHP 5.3.0
A quick solution will be is to open the file modules/mod_mainmenu/helper.php in your root folder and change the following line from
at the top of helper.php
The best solution will be to upgrade your Joomla! installation to the latest Joomla! release
Image Credits: Photo by Isis França on Unsplash.