Symfony is a rather popular PHP framework; it is commonly regarded in the PHP community as one of the higher-quality frameworks (amidst many half-baked frameworks that cause serious problems once your use case moves past the blog tutorial).

In terms of functionality, it's basically for PHP what Rails is for Ruby, but unlike many other PHP frameworks, it's been designed with the platform in mind rather than trying to force Rails' concepts into PHP, as a result if which it feels "just better". This has given it an edge over competitors, which is why Symfony is remarkably popular among the more professional PHP businesses.

I think it's basically the only mature and popular surviving framework that dates from the blast of PHP frameworks that came to be when Rails got popular and PHP coders wanted that too.