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Basic usage


To install Composer, you just need to download the composer.phar executable.

$ curl -s | php

For the details, see the Introduction chapter.

To check if Composer is working, just run the PHAR through php:

$ php composer.phar

This should give you a list of available commands.

Note: You can also perform the checks only without downloading Composer by using the --check option. For more information, just use --help.

$ curl -s | php -- --help

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 JSON format is quite easy to write. It allows you to define nested structures.

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 package versions (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

We are requiring version 1.0.* of monolog. This means any version in the 1.0 development branch. It would match 1.0.0, 1.0.2 or 1.0.20.

Version constraints can be specified in a few different ways.

  • Exact version: You can specify the exact version of a package, for example 1.0.2. This is not used very often, but can be useful.

  • Range: By using comparison operators you can specify ranges of valid versions. Valid operators are >, >=, <, <=. An example range would be >=1.0. You can define multiple ranges, separated by a comma: >=1.0,<2.0.

  • Wildcard: You can specify a pattern with a * wildcard. 1.0.* is the equivalent of >=1.0,<1.1-dev.

Installing Dependencies

To fetch the defined dependencies into your local project, just run the install command of composer.phar.

$ php composer.phar install

This will find the latest version of monolog/monolog that matches the supplied version constraint and download it into the 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 into your .gitignore. You really don't want to add all of that code to your repository.

Another thing that the install command does is it adds a composer.lock file into your project root.

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.

If no composer.json lock file exists, it will read the dependencies and versions from composer.json and create the lock file.

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 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: For libraries it is not necessarily recommended 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 should publish their packages on packagist. A library doesn't need to be on packagist to be used by Composer, but it makes life quite a bit simpler.


For libraries that follow the PSR-0 naming standard, Composer generates a vendor/.composer/autoload.php file for autoloading. You can simply include this file and you will get autoloading for free.

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

This makes it really easy to use third party code: For monolog, it means that we 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-0": {"Acme": "src/"}

This is a mapping from namespaces to directories. The src directory would be in your project root. An example filename would be src/Acme/Foo.php containing an Acme\Foo class.

After adding the autoload field, you have to re-run install to re-generate the vendor/.composer/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 'vendor/.composer/autoload.php';
$loader->add('Acme\Test', __DIR__);

Note: Composer provides its own autoloader. If you don't want to use that one, you can just include vendor/.composer/autoload_namespaces.php, which returns an associative array mapping namespaces to directories.

Intro | Libraries

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