Basic usage#


For our basic usage introduction, we will be installing monolog/monolog, a logging library. If you have not yet installed Composer, refer to the Intro chapter.

Note: for the sake of simplicity, this introduction will assume you have performed a local install of Composer.

composer.json: Project Setup#

To start using Composer in your project, all you need is a composer.json file. This file describes the dependencies of your project and may contain other metadata as well.

The require Key#

The first (and often only) thing you specify in composer.json is the require key. You're simply telling Composer which packages your project depends on.

    "require": {
        "monolog/monolog": "1.0.*"

As you can see, require takes an object that maps package names (e.g. monolog/monolog) to version constraints (e.g. 1.0.*).

Package Names#

The package name consists of a vendor name and the project's name. Often these will be identical - the vendor name just exists to prevent naming clashes. It allows two different people to create a library named json, which would then just be named igorw/json and seldaek/json.

Here we are requiring monolog/monolog, so the vendor name is the same as the project's name. For projects with a unique name this is recommended. It also allows adding more related projects under the same namespace later on. If you are maintaining a library, this would make it really easy to split it up into smaller decoupled parts.

Package Versions#

In the previous example we were requiring version 1.0.* of Monolog. This means any version in the 1.0 development branch. It is the equivalent of saying versions that match >=1.0 <1.1.

Version constraints can be specified in several ways, read versions for more in-depth information on this topic.


By default only stable releases are taken into consideration. If you would like to also get RC, beta, alpha or dev versions of your dependencies you can do so using stability flags. To change that for all packages instead of doing per dependency you can also use the minimum-stability setting.

Installing Dependencies#

To install the defined dependencies for your project, just run the install command.

php composer.phar install

This will find the latest version of monolog/monolog that matches the supplied version constraint and download it into the vendor directory. It's a convention to put third party code into a directory named vendor. In case of Monolog it will put it into vendor/monolog/monolog.

Tip: If you are using git for your project, you probably want to add vendor in your .gitignore. You really don't want to add all of that code to your repository.

You will notice the install command also created a composer.lock file.

composer.lock - The Lock File#

After installing the dependencies, Composer writes the list of the exact versions it installed into a composer.lock file. This locks the project to those specific versions.

Commit your application's composer.lock (along with composer.json) into version control.

This is important because the install command checks if a lock file is present, and if it is, it downloads the versions specified there (regardless of what composer.json says).

This means that anyone who sets up the project will download the exact same version of the dependencies. Your CI server, production machines, other developers in your team, everything and everyone runs on the same dependencies, which mitigates the potential for bugs affecting only some parts of the deployments. Even if you develop alone, in six months when reinstalling the project you can feel confident the dependencies installed are still working even if your dependencies released many new versions since then.

If no composer.lock file exists, Composer will read the dependencies and versions from composer.json and create the lock file after executing the update or the install command.

This means that if any of the dependencies get a new version, you won't get the updates automatically. To update to the new version, use the update command. This will fetch the latest matching versions (according to your composer.json file) and also update the lock file with the new version.

php composer.phar update

Note: Composer will display a Warning when executing an install command if composer.lock and composer.json are not synchronized.

If you only want to install or update one dependency, you can whitelist them:

php composer.phar update monolog/monolog [...]

Note: For libraries it is not necessary to commit the lock file, see also: Libraries - Lock file.


Packagist is the main Composer repository. A Composer repository is basically a package source: a place where you can get packages from. Packagist aims to be the central repository that everybody uses. This means that you can automatically require any package that is available there.

If you go to the Packagist website (, you can browse and search for packages.

Any open source project using Composer is recommended to publish their packages on Packagist. A library doesn't need to be on Packagist to be used by Composer, but it enables discovery and adoption by other developers more quickly.


For libraries that specify autoload information, Composer generates a vendor/autoload.php file. You can simply include this file and you will get autoloading for free.

require __DIR__ . '/vendor/autoload.php';

This makes it really easy to use third party code. For example: If your project depends on Monolog, you can just start using classes from it, and they will be autoloaded.

$log = new Monolog\Logger('name');
$log->pushHandler(new Monolog\Handler\StreamHandler('app.log', Monolog\Logger::WARNING));

You can even add your own code to the autoloader by adding an autoload field to composer.json.

    "autoload": {
        "psr-4": {"Acme\\": "src/"}

Composer will register a PSR-4 autoloader for the Acme namespace.

You define a mapping from namespaces to directories. The src directory would be in your project root, on the same level as vendor directory is. An example filename would be src/Foo.php containing an Acme\Foo class.

After adding the autoload field, you have to re-run dump-autoload to re-generate the vendor/autoload.php file.

Including that file will also return the autoloader instance, so you can store the return value of the include call in a variable and add more namespaces. This can be useful for autoloading classes in a test suite, for example.

$loader = require __DIR__ . '/vendor/autoload.php';
$loader->addPsr4('Acme\\Test\\', __DIR__);

In addition to PSR-4 autoloading, Composer also supports PSR-0, classmap and files autoloading. See the autoload reference for more information.

Note: Composer provides its own autoloader. If you don't want to use that one, you can just include vendor/composer/autoload_*.php files, which return associative arrays allowing you to configure your own autoloader.

Intro | Libraries

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