• Not a package manager
  
• Dependency manager
  
• Based on NPM & Ruby's Bundler
  
• Why not use PEAR?

• Download PHP libraries/packages
  your project depends on
• Specify version of each dependency 
  (using semantic versioning)
• Download dependencies of your dependencies
• Writes autoloader file and class map
   of dependencies
• Supports PSR-0, but not required
• Creates projects based on packages*
• Can use multiple repositories

{
    "require": {
        "vendor/package": "1.3.2",
    }
}

• Require line composed of
  vendor name, package name, & version
• Needs to be in the root of the project
   along with composer.phar
• Can also specify "require-dev" which will only
  be installed if the --dev flag is passed to the install
  command or --no-dev is omitted from update

• Version can be exact as above
• Can request specific stability level

  {
      "require": {
          "monolog/monolog": "1.0.*@beta",
          "acme/foo": "@dev"
      }
  }
  

• Stability: @dev, @alpha, @beta, @rc, & @stable (default)
• Can request specific commits via #<ref>

  {
      "require": {
          "acme/foo": "1.0.x-dev#abc123"
      }
  }

• X.Y.* - Match the latest stable minor version
  (i.e. 1.2.3 will be selected rather than 1.2.2)
• >= X.Y.Z, < A.B.C - Match a range of versions
  (i.e. any stable version between 1.2.3 & 2.0.0)
• ~X.Y.Z - Match next significant release
  (i.e. ~1.2 will match up to 1.9, ~1.1.3 will match 1.1.99)

• Certain packages can be used to create projects
  

  php composer.phar create-project -sdev 
  --repository-url="http://packages.zendframework.com" 
  zendframework/skeleton-application path/to/install

This is the equivalent of doing a git clone/svn checkout followed by a composer install of the vendors.

• Or clone the project and run
  php composer.phar install
• Dependencies will be installed in the vendor directory

• The composer.lock file is created after
  the first install command is run
• It locks the project to the specific version
  of the dependencies installed
• If lock file is present, install command will ignore
  json file and install versions from lock file
• Composer.lock file should be commited to VCS

composer.json
    {
        "require" : {
            "aws/aws-sdk-php-zf2": "1.0.*"
        }
    }

  php composer.phar install

composer.lock contains
    {
        "require" : {
            "aws/aws-sdk-php-zf2": "1.0.1"
        }
    }

• All future clones of the project will install 
  version 1.0.1
• To upgrade a dependency run

php composer.phar update

• Update command installs the latest version
• Changes the installed version on the lock file
  
• Lock file should be recommitted
  along with composer.json

• Install command creates autoloader
• Include with
  

  require "vendor/autoload.php";

• Package must be on a VCS repository
  (GitHub, BitBucket, private etc.)
• Repository does not have to be public
  as long as the web user has
  access to the repository
• Packagist crawls the repository for branches
  and the composer.json file
• The information gets displayed on the package page

• To Packagist, all projects are unnamed
  libraries until they have a name
• Names cannot change
• Need to be unique enough to avoid conflicts
• Name should be composed of vendor &
  package name separated by "/"

• Virtual packages that exists in the system
  but are not installable by Composer
  
• Can constrain versions
• "php" key references version of PHP
• "ext-<name>" references PHP extensions
• "lib-<name>" references libraries PHP can use
  such as curl

• Versions detected through branch names
  or tags
• The version key on the json file is
  not recommended
• For versions to be used must conform to
  Semantic Versioning
• Leverage stability flags on tags for users
  (i.e. 2.3.0-alpha1)

• PSR-0 autoloading for classes by defining
  root path
  

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

• Classmap autoloading by defining include
  directories

  {
      "autoload": {
          "classmap": ["src/", "lib/", "MyClass.php"]
      }
  }
  

• Specific file autoloading

  {
      "autoload": {
          "files": ["src/MyLibrary/functions.php"]
      }
  }