# Basic usage

## Installation

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

    $ curl -sS https://getcomposer.org/installer | php

For the details, see the [Introduction](00-intro.md) 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 -sS https://getcomposer.org/installer | 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](http://json.org/) 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`.

* **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`.

* **Next Significant Release (Tilde Operator):** The `~` operator is best
  explained by example: `~1.2` is equivalent to `>=1.2,<2.0`, while `~1.2.3` is
  equivalent to `>=1.2.3,<1.3`. As you can see it is mostly useful for projects
  respecting semantic versioning. A common usage would be to mark the minimum
  minor version you depend on, like `~1.2`, since in theory there should be no
  backwards compatibility breaks until 2.0, that works well.

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](04-schema.md#package-links). To change that for all
packages instead of doing per dependency you can also use the
[minimum-stability](04-schema.md#minimum-stability) setting.

## 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 `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.lock` file exists, Composer 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](02-libraries.md#lock-file).

## Packagist

[Packagist](https://packagist.org/) 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](https://packagist.org/) (packagist.org),
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.

## Autoloading

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 '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));

    $log->addWarning('Foo');

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

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

Composer will register a
[PSR-0](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md)
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/Acme/Foo.php` containing an `Acme\Foo` class.

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

In addition to PSR-0 autoloading, classmap is also supported. This allows
classes to be autoloaded even if they do not conform to PSR-0. See the
[autoload reference](04-schema.md#autoload) for more details.

> **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.

&larr; [Intro](00-intro.md)  |  [Libraries](02-libraries.md) &rarr;