# Basic usage

## Installation

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

    $ curl -s http://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 -s http://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`. 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 project'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 gets a new version, you won't be updated
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

## Packagist

[Packagist](http://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](http://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 follow the [PSR-0](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md)
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', 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/"}
        }
    }

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.

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