The Flex composer.json File

Question: What are these lines doing ?
`
"symfony": {

        "allow-contrib": true
    }

`

Question: We are currently at Symfony 4.3. Will the tutorial work if I use the composer.json from that branch? Or should I use this <a href="https://github.com/symfony/skeleton/blob/4.0/composer.json&quot;&gt;composer.json&lt;/a&gt; from the 4.0 branch?

Also, this file seems to be structured a bit differently now. At some point Ryan says to copy the "require" section of composer.json to get the framework-bundle. But in the latest composer.json, the framework-bundle is in a separate section called "flex-require." I tried copying both the "require" and "flex-require" sections, but I then get an error with composer update.

After this video I refreshed my project in the browser and this is what I've got.
`

Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0

Fatal error: Unknown: Failed opening required '..../vendor/symfony/symfony/src/Symfony/Bundle/WebServerBundle/Resources/router.php' (include_path='.:/usr/local/php5/lib/php') in Unknown on line 0
`

After googling I found out that I have exactly this issue: https://github.com/symfony/symfony/issues/25515

But I am now stuck because I have no idea what needs to be done and I am kind of useless with composer and github generally.

Everything whats in this video I have done and saw same messages on my mac but could someone point me what should I do.
Also please explain it like I was 6yrs old.

my composer.json if thats needed.

`
{

"name": "peterkosak/myproject",
"license": "proprietary",
"type": "project",
"autoload": {
    "psr-4": {
        "AppBundle\\": "src/AppBundle",
        "App\\": "src/"
    },
    "classmap": [
        "app/AppKernel.php",
        "app/AppCache.php"
    ]
},
"autoload-dev": {
    "psr-4": {
        "Tests\\AppBundle\\": "tests/AppBundle",
        "App\\Tests\\": "tests/"
    },
    "files": [
        "vendor/symfony/symfony/src/Symfony/Component/VarDumper/Resources/functions/dump.php"
    ]
},
"require": {
    "php": "^7.1.3",
    "ext-iconv": "*",
    "symfony/console": "^4.0",
    "symfony/flex": "^1.0",
    "symfony/framework-bundle": "^4.0",
    "symfony/lts": "^4@dev",
    "symfony/yaml": "^4.0",
    "doctrine/doctrine-bundle": "^1.6",
    "doctrine/doctrine-migrations-bundle": "^1.2",
    "doctrine/orm": "^2.5",
    "sensio/framework-extra-bundle": "^3.0.2",
    "symfony/monolog-bundle": "^3.1.0",
    "symfony/polyfill-apcu": "^1.0",
    "symfony/swiftmailer-bundle": "^3.1",
    "twig/twig": "^1.0||^2.0"
},
"require-dev": {
    "symfony/dotenv": "^4.0",
    "symfony/thanks": "^1.0",
    "doctrine/doctrine-fixtures-bundle": "^2.4",
    "symfony/phpunit-bridge": "^4.0"
},
"config": {
    "preferred-install": {
        "*": "dist"
    },
    "sort-packages": true
},
"replace": {
  "symfony/polyfill-iconv": "*",
  "symfony/polyfill-php71": "*",
  "symfony/polyfill-php70": "*",
  "symfony/polyfill-php56": "*"
},
"scripts": {
    "auto-scripts": {
        "cache:clear": "symfony-cmd",
        "assets:install --symlink --relative %PUBLIC_DIR%": "symfony-cmd"
    },
    "post-install-cmd": [
        "@auto-scripts"
    ],
    "post-update-cmd": [
        "@auto-scripts"
    ]
},
"conflict": {
    "symfony/symfony": "*"
},
"extra": {
    "symfony": {
        "id": "01C1ZF996JGZVMYFK23DZBEG0A",
        "allow-contrib": false
    }
}

}

`

Hi Guys,

for beginners be very careful here. I am new to symfony, composer and console. I was trying to develop my old project that wasnt using any framework to symfony. I was skipping tutorials based on what I needed. Everything was working in symfony version 3 what I learned and I was really happy how easy it is.

Then I wanted to upgrade to symfony 4 so first thing that will fail is your build in webserver that will run 5.6.30 on sierra so you need to upgrade it to run composer I think. Thats probably where things get messed up. Because my php -v still showed 5.6.30 however I installed 7.2 somehow I managed to show 7.2 in terminal after I typed php -v so I continue to watched this video/tutorial. Today I managed to broke my whole project when running composer update. I cant even run php bin/console server:run that was working on symfony 3 version because bin/console didnt even exist so be very carefull I have no idea what I did because I was trying to fix it and probably broke other 10 thinks. Lesson learned for me to run backups and I can start from scratch.

Also this is probably why I do not trust framework. It is so easy to to break things. When you are not using framework and you will remove file you know it will break something but here I have no idea what will happen when I delete 1 file somewhere deeeeep in vendor folder.

Well at least it will force me to wait and watch symfony 4 tutorial here so I can start again.

Same here, application broken !
At the end I think it was a false good idea to start this course before it was completly published.
Ha ha ha !!
So I'm just gonna wait and pray for a quick release.
Cheers up KNP team ;)