Have an awesome library you want to integrate in your Symfony application? Share your code between different Symfony instances? Share your code with other people? Create your bundle standalone and distribute it via Composer! Here is a short overview on how to do it.

To get started, you want to have a Composer & a git instance (if you follow the general practice to publish your bundle on a git-based central repository, such as GitHub or GitLab) running. All a Symfony third-party bundle has to be is a composer package. Everything else is bonus. To make your project a composer package, you add the composer.json file if you did not already. In it, you specify the name, properties and dependencies of your bundle. Comparsion with other bundles helps to get an impression of what to put in there. Basically, it could look like mine for my PlaceholderBundle:

{
    "name" : "bernhard-webstudio/placeholder-bundle",
    "type" : "symfony-bundle",
    "description" : "Symfony bundle to generate placeholders for images",
    "keywords" : ["Symfony", "Placeholder"],
    "homepage" : "https://github.com/BernhardWebstudio/PlaceholderBundle",
    "license" : "MIT",
    "authors" :
    [
        {
            "name" : "Tim Bernhard",
            "email" : "tim@bernhard-webstudio.ch",
            "homepage" : "http://genieblog.ch",
            "role" : "Developer"
        }
    ],

After adding the boni, you will want to publish your bundle so other people can benefit from your bundle. There are various tutorials for this already so I will not write on.

Boni

To give the Symfony character to your package, you will want to integrate e.g. dependency injection, expose a class as a service or use the general Symfony configuration for your services. This can be achieved by adding the relevant Symfony components as dependencies via composer, e.g. use command composer require symfony/dependency-injection. Afterwards, you can use them the same way you are using them in your Symfony application. Pay attention not to limit the dependency versions too much, otherwise you may prevent a user from upgrading Symfony.

There are may boni you can add to your repository, if it is published on e.g. GitHub: to give users and contributors a chance to better understand the state of your repository, integrate some of the apps and add badges to your README file.

Finally, let's talk testing: as your package should not ship with a Kernel oder similar, you have nothing to run. There are workarounds: either you let your bundle live in a complete application, e.g. as a submodule, and test the application as a whole. Or you use unit-testing. If your choice falls on the latter, I can recommend using ConfigTest if your bundle uses the configuration component. Also, at one point, you may have to have an AppKernel to test the execution of a command or the availability of a route. Depending on the actual need, you can either mock the Kernel or create a TestKernel. For latter are many examples online in other bundles, e.g. in mine.