To install Doctrine, we need to create a file called composer.json in our new blog directory. It lists dependencies of our project as shown in the following code:

{
    "name": "myname/blog",
    "type": "project",
    "description": "My small blog to play with Doctrine",

    "require": {
        "doctrine/orm": "2.4.*"
    },

    "autoload": {
        "psr-0": { "": "src/" }
    }
} 

This standard JSON file will be parsed by Composer to download and install all dependencies specified. Once installed, Composer will load all classes of these libraries automatically.

The name, type, and description attributes are optional but it's a good practice to always fill them. They provide general information about the project we are working on.

The more interesting part of this composer.json file is the require field. In order to get it installed by Composer, all libraries used by our app must be listed here. A lot of PHP libraries are available on Packagist, the default Composer package repository. Of course, it's the case of Doctrine projects.

We indicate that we need the latest minor release of the 2.4 branch of Doctrine ORM. You can set a major or minor version here, and even more complicated things.

The autoload field is here to tell Composer to automatically load classes of our app. We will put our specific code in a directory called src/. Our files and classes will follow the PSR-0 namespacing and file-naming standard.

It's time to use Composer to install the ORM. Run the following command:

    php composer.phar install

New files appear in the vendor/ directory. Doctrine ORM has been installed, and Composer was smart enough to get all its dependencies, including Doctrine DBAL and Doctrine Common.

A composer.lock file has also been created. It contains exact versions of installed libraries. This is useful for deploying applications. Thanks to this file, when running the install command, Composer will be able to retrieve the same versions that have been used in the development.

Doctrine is now properly installed. Easy, isn't it?

To update libraries when there are new releases in the 2.4 branch, we just need to type the following command:

    php composer.phar update