I originally was using Mustache.js but found that it does not work well for rendering large nested trees (it has no way of disambiguating variables of the same name in nested structures).
I was happy to find a PHP version of Underscore.js, but when I looked at the code of Underscore.php I realized that its template method does not render Underscore.js-style templates. Instead it replicates similar functionality but with PHP variables.
Jquery-tmpl/jquery-tmpl-php is another template language with JS and PHP libraries, but my concerns are that the jquery-tmpl-php library seems not to be used much (very few people are following it on github) and that jQuery decided to remove jQuery-tmpl as an official plugin: http://blog.jquery.com/2011/04/16/official-plugins-a-change-in-the-roadmap/
Also it seems that the author of jquery-tmpl has not touched it in months.
What are other people doing to render Javascript and PHP using the same templates?
Jade does it:
https://github.com/everzet/jade.php
https://github.com/visionmedia/jade