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 are 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.*).

Composer uses this information to search for the right set of files in package "repositories" that you register using the repositories key, or in Packagist, the default package respository. In the above example, since no other repository has been registered in the composer.json file, it is assumed that the monolog/monolog package is registered on Packagist. (See more about Packagist below, or read more about repositories here).

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. For example, it would allow two different people to create a library named json. One might be named igorw/json while the other might be seldaek/json.

Read more about publishing packages and package naming here. (Note that you can also specify "platform packages" as dependencies, allowing you to require certain versions of server software. See platform packages below.)

Package Version Constraints#

In our example, we are requesting the Monolog package with the version constraint 1.0.*. This means any version in the 1.0 development branch, or any version that is greater than or equal to 1.0 and less than 1.1 (>=1.0 <1.1).

Please read versions for more in-depth information on versions, how versions relate to each other, and on version constraints.

How does Composer download the right files? When you specify a dependency in composer.json, Composer first takes the name of the package that you have requested and searches for it in any repositories that you have registered using the repositories key. If you have not registered any extra repositories, or it does not find a package with that name in the repositories you have specified, it falls back to Packagist (more below).

When Composer finds the right package, either in Packagist or in a repo you have specified, it then uses the versioning features of the package's VCS (i.e., branches and tags) to attempt to find the best match for the version constraint you have specified. Be sure to read about versions and package resolution in the versions article.

Note: If you are trying to require a package but Composer throws an error regarding package stability, the version you have specified may not meet your default minimum stability requirements. By default only stable releases are taken into consideration when searching for valid package versions in your VCS.

You might run into this if you are trying to require dev, alpha, beta, or RC versions of a package. Read more about stability flags and the minimum-stability key on the schema page.

Installing Dependencies#

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

php composer.phar install

When you run this command, one of two things may happen:

Installing Without composer.lock#

If you have never run the command before and there is also no composer.lock file present, Composer simply resolves all dependencies listed in your composer.json file and downloads the latest version of their files into the vendor directory in your project. (The vendor directory is the conventional location for all third-party code in a project). In our example from above, you would end up with the Monolog source files in vendor/monolog/monolog/. If Monolog listed any dependencies, those would also be in folders under vendor/.

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 third-party code to your versioned repository.

When Composer has finished installing, it writes all of the packages and the exact versions of them that it downloaded to the composer.lock file, locking the project to those specific versions. You should commit the composer.lock file to your project repo so that all people working on the project are locked to the same versions of dependencies (more below).

Installing With composer.lock#

This brings us to the second scenario. If there is already a composer.lock file as well as a composer.json file when you run composer install, it means either you ran the install command before, or someone else on the project ran the install command and committed the composer.lock file to the project (which is good).

Either way, running install when a composer.lock file is present resolves and installs all dependencies that you listed in composer.json, but Composer uses the exact versions listed in composer.lock to ensure that the package versions are consistent for everyone working on your project. As a result you will have all dependencies requested by your composer.json file, but they may not all be at the very latest available versions (some of the dependencies listed in the composer.lock file may have released newer versions since the file was created). This is by design, it ensures that your project does not break because of unexpected changes in dependencies.

Commit Your composer.lock File to Version Control#

Committing this file to VC is important because it will cause anyone who sets up the project to use the exact same versions of the dependencies that you are using. 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. (See note below about using the update command.)

Updating Dependencies to their Latest Versions#

As mentioned above, the composer.lock file prevents you from automatically getting the latest versions of your dependencies. To update to the latest versions, use the update command. This will fetch the latest matching versions (according to your composer.json file) and update the lock file with the new versions. (This is equivalent to deleting the composer.lock file and running install again.)

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, without further specifying where Composer should look for the package.

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 does not need to be on Packagist to be used by Composer, but it enables discovery and adoption by other developers more quickly.

Platform packages#

Composer has platform packages, which are virtual packages for things that are installed on the system but are not actually installable by Composer. This includes PHP itself, PHP extensions and some system libraries.

You can use show --platform to get a list of your locally available platform packages.


For libraries that specify autoload information, Composer generates a vendor/autoload.php file. You can simply include this file and start using the classes that those libraries provide without any extra work:

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

$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.

See also the docs on optimizing the autoloader.

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.

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