How recover composer.json by existing project on symfony2?

You should create bare composer.json by hand or using command composer init.

And then you can list all the packages under the vendor folder by composer show --installed.

Then just generate require section for your composer.json with listed values. And you are done. You can use regular expressions to do it easier.

composer show --installed \
   | awk '{printf "\"%s\": \"^%s\",\n", $1, $2}' \
   | sed -r 's:\^v:^:g' \
   >> packages.list