Symfony [SF] is probably on a par with CodeIgniter [CI] in terms of its use [circa 7-8%] and is a very stable framework. It has lost of a lot of ground to Laravel and in non-tech terms it's really just because the two are pretty similar and Laravel requires less configuration than SF**. Some people prefer `this`, we don't but it makes little difference in the long-run.
SF is really the traditional choice of the corporate and if you love PHP testing and buzz words like ORM [Doctrine] and Entities are your thing then SF will be for you.
SF would not be our first choice, but just like CI, if this is your platform of choice and you'll have good reasons for this then SF is a fine MVC framework to use. Laravel is easier to use, but that's just because it comes pre-bundled with a lot of `stuff` and Laravel is built on SF so it's not that there is really a major difference here, just that Laravel is a bit of a `sugar` over SF and a good one.
Going with SF is a bit like the old aphorism of "No one ever got fired for buying IBM" and SF is used by some very large corporations, but the end results are almost indistinguishable from Laravel in production. If testing, auditing and the word `PHPUnit` means a lot to you then SF is a good choice. **Just be aware that it's more of a platform you add to [which can be good] whereas Laravel has sort of done this for you.
2020: It is also worthy of note that with PHP 7.4.x and some benchmarks of Laravel vs SF, that dear ole SF has not faired well! In fact Laravel | SF with a standard mySQL database was hands-down Laravel a winner with it outperforming SF by a factor x3 - Yes, SF was three times slower! This was tested on a LAMP stack with an i7 SSD fast server making 1,000 DB writes and SF was frankly weak. Some SF fans will tell you that this performance can be adjusted, but try telling that to your public Web Host ? In summary, use Laravel and dump SF. On a coding note: use Blade and not Twig? Laravel dropped Twig like a stone and so should you.
It is frankly odd to deploy SF for new projects when Laravel is clearly far superior and berate Vadoo all you like, but the real web world is now using Laravel and not SF. Unless you work for a Bank or Insurance company and your hands are tied - Sorry about that. Any good Laravel coder will work with SF and vice-versa, but in 2020 your reasons for chosing SF over Laravel would be, at best, questionable?
As a final rant, some PHP - all in one - solutions love to deploy vue.js and dear ole jQuery is a waste of time. Fair enough that jQuery is a tad outdated, but if you are serious: refactor to a front and backend with React or Angular. Your API is what matters. Your backend dashboard will be used by your staff and not the world.