Receiving PHP Warning on move to Production
So Brad tracked down the issue being Twig needs at least PHP 5.2.7 to run. As of 1.34, the minimum requirement was bumped to PHP 5.3.3. Craft bumped up Twig to 1.35 in version 2.6.3001

https://twig.symfony.com/doc/1.x/intro.html

My clients server was actually running 5.3.29. The server default was 7, bumping it up to php 5.4 fixed the site.