Except for the ones that you can't fix, because they're not coming from your code.
It's pretty likely that you have a group of external libraries and bundles that are
also causing deprecation warnings.
In our app, we have this exact problem, with AsseticBundle. So before we upgrade
to Symfony 3, we need to upgrade to a new version of that library that does not
use old code.
No problem! Google for AsseticBundle. I'll click into the link on Packagist.
I'm looking here to find the latest released version of Packagist, which right now,
is 2.8.0. If we update to that, those deprecation warnings will hopefully go away.
In general, if a library is giving you deprecation warnings like this one, you do
need to upgrade it, but you may not need to upgrade it to the absolute latest
version. If you want to be a bit more conservative, you can dig into the release
notes of a library to find the first version number that is Symfony 3 compatible.
It's more work, but might mean less QA time than upgrading a library many versions
all at once.
Hello AsseticBundle 2.8.0! Oh, and see those messages from Composer? That's another
usage of deprecations in action: some of our code is using some deprecated functionality
in Composer itself. Those will probably go away when we upgrade Symfony. But, either
way, it's not important right now.
Let's check in on our deprecations: refresh the page. Yes! 33 deprecations down to
one. And don't worry about that last deprecation - it's really specific to this
project - so I'm not going to take time to fix it.
Ok, we've now upgraded all outside libraries that were not Symfony 3 compatible
and our deprecation warnings are gone. We are ready.