This integration tested on Zend Framework 1.11.10, Twig 1.20 and PHP 5.3.8.

• Download Twig and place contents of lib/Twig folder to /path/to/app/library/Twig.
• Place App folder to /path/to/app/library folder.
• Update files of /path/to/app/application/views/scripts with application/views/scripts files.

Add following lines to your application.ini file.

[production]    
autoloaderNamespaces[] = "App"
autoloaderNamespaces[] = "Twig"
pluginPaths.App_Resource_ = "App/Resource/"
resources.twig.cache = "/path/to/cache/folder"

[development : production]
resources.twig.debug = true

Note : This integration changes your view filenames extension to twig instead of phtml.