This article may seems to you extra, as the topic has been covered in many places. There are many other and preferred ways to do this. The reason why I chose the topic, is to show what are the difficulties I have faced while migrating one project. So...hang tight: From Symfony 3.x, the symfony installaer came into light. So to install a Symfony 3.4 version you can opt for any 02 paths: 1\. Using installer: Grab Symfony installer
> https://github.com/symfony/symfony-installer c:\bin\> php -r "file_put_contents('symfony', file_get_contents('https://symfony.com/installer'));" c:\bin\> (echo @ECHO OFF & echo php "%~dp0symfony" %*) > symfony.bat
Install Symfony 3.4 cd to htdocs(I used xampp) and run
> symfony new your_project_name 3.4
2\. Using composer:
> composer create-project symfony/framework-standard-edition your_project_name/ "3.4.*"
Now once to change directory to your_project_name, first thing you have to do is to create bundle. You might have installed custom bundle in your Symfony 2.x phase:
> php bin/console generate:bundle --namespace=YourNameSpace/YourBundle
This will ask several inputs, only value to provide is configuration format which should be yml (you may need to try twice) 
Now you will face and autoload issue: It will not able to change autoload and the message below will be displayed: The command was not able to configure everything automatically. 
Now open composer.json and locate the autoload section: 
Change it to "psr-4": { "": "src/" }, Which will looks like: 
Now run composer update, see what happened in following figure: 
Acquire dependent bundles Now we must acquire all dependent bundles, need to run
> composer require sonata-project/doctrine-orm-admin-bundle
and so on..... While configure, you will notice many deprecation issue. It depends on bundle configurations. For an example: To remove deprecation, found this https://stackoverflow.com/questions/47698006/symfony-3-4-refreshing-a-deauthenticated-user-is-deprecated, security.yml should looks like:
> main: logout_on_user_change: true anonymous: ~
Import & configure DB Now it’s time to import database: Run
> php bin/console doctrine:database:create --connection=default
This will create database, now import the existing database into newly created one Now copy doctrine folder and paste into src/yourbundle/Resources/config folder run
> php bin/console doctrine:generate:entities YourNameSpace/YourBundle php bin/console doctrine:schema:update –force
Database is configured, now setup vhost and try in browser (add app_dev.php in .htaccess and comment line #13-19 in app_dev.php) Now copy all folders from bundle, except DependencyInjection, Entity, Resources etc. Move views and public assets Copy src/Bundle/Resources/public directory to same location in Symfony 3.4 and run below command
> php bin/console assets:install web php bin/console cache:clear --env=dev
Copy existing src/Bundle/Resources/views files to app/Resources/views Remove all YourNameSpaceYourBundle::” bundle scope from twigs as all templates moved to app/Resources from src/Bundle namespace That's all as of now. Stay tuned !!!