It's as easy as 4 steps (minus the part where you code a reusable package in PHP) -- I was surprised too. Make sure before starting you've created a git repository in your project and committed your code. It's also required to have an account on [Github](http://github.com/) and [Packagist](http://packagist/).

1. Create a `composer.json` in your project:
    
    ```json
    {
        "name": "your-brand-name/your-project",
        "type": "library",
        "description": "Your package description goes here",
        "keywords": ["relevant","tags","go","here"],
        "homepage": "<https://yourcompany.com>",
        "license": "MIT",
        "authors": [
            {
                "name": "Jordi Boggiano",
                "email": "j.boggiano@seld.be",
                "homepage": "<http://seld.be>",
                "role": "Developer"
            }
        ],
    }
    ```
    
2. [Create a new repository on your Github](https://github.com/new) and push your local git (init if you haven't) to the remote Github repo.
3. [Submit your package to the Packagist](https://packagist.org/packages/submit)
4. Done! Your package should be online and Packagist should provide a sample Composer require with your project name (e.g. `composer require username/package-name`).  
    You should be able to view your package live at:  
    [https://packagist.org/packages/your-username/package-name](https://packagist.org/packages/your-username/package-name)