PHP-DI is an easy to integrate IoC container that you can use in legacy projects or in apps without a framework.

I regularly have to use Zend Framework 1 for some legacy apps.  If you're used to having an IoC container to inject dependencies then not having one is a big pain. Here's a guide on how to integrate it in ZF1:

http://php-di.org/doc/frameworks/zf1.html