I am working on \"modernizing\" a pre-existing PHP-driven website. This website started out as a static website with a few php methods. It now has a mobile
Liek hakre said, Assetic works out of the box. You do not need any templating system (Twig, Smarty...) for it works.
With a templating system, Twig is the best because, like hakre said, it's strongly implemented. However, it's not that diffucult to integrate it into any other template system (I quickly wrote a Smarty plugin for personnal use, it works well).
Finaly Assetic does anything you need :
The last advantage that goes to Assetic is that it's the one used by default in Symfony2, which is, imo, one of the best PHP framework lately, so it proves that Assetic is a good choice.
Only problem I had so far using Assetic out of Symfony was the lack of documentation (the usage of the CssRewriteFilter in my case) but good understanding of the source code often helps getting how it's supposed to work.