curl -sS https://getcomposer.org/installer | php
mv composer.phar /usr/local/bin/composer

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Composer version 1.0-dev (4f80e7ff68466cab970c22b425725b8058c32970) 2015-06-09 00:03:48

Usage:
command [options] [arguments]

Options:
--help (-h)           Display this help message
--quiet (-q)          Do not output any message
--verbose (-v|vv|vvv) Increase the verbosity of messages: 1 for normal output, 2 for more verbose output and 3 for debug
--version (-V)        Display this application version
--ansi                Force ANSI output
--no-ansi             Disable ANSI output
--no-interaction (-n) Do not ask any interactive question
--profile             Display timing and memory usage information
--working-dir (-d)    If specified, use the given directory as working directory.

Available commands:
about            Short information about Composer
archive          Create an archive of this composer package
browse           Opens the package's repository URL or homepage in your browser.
clear-cache      Clears composer's internal package cache.
clearcache       Clears composer's internal package cache.
config           Set config options
create-project   Create new project from a package into given directory.
depends          Shows which packages depend on the given package
diagnose         Diagnoses the system to identify common errors.
dump-autoload    Dumps the autoloader
dumpautoload     Dumps the autoloader
global           Allows running commands in the global composer dir ($COMPOSER_HOME).
help             Displays help for a command
home             Opens the package's repository URL or homepage in your browser.
info             Show information about packages
init             Creates a basic composer.json file in current directory.
install          Installs the project dependencies from the composer.lock file if present, or falls back on the composer.json.
licenses         Show information about licenses of dependencies
list             Lists commands
remove           Removes a package from the require or require-dev
require          Adds required packages to your composer.json and installs them
run-script       Run the scripts defined in composer.json.
search           Search for packages
self-update      Updates composer.phar to the latest version.
selfupdate       Updates composer.phar to the latest version.
show             Show information about packages
status           Show a list of locally modified packages
update           Updates your dependencies to the latest version according to composer.json, and updates the composer.lock file.
validate         Validates a composer.json

composer require --prefer-dist yiisoft/yii2-imagine

"require": {
      "php": ">=5.4.0",
      "yiisoft/yii2": ">=2.0.4",
      "yiisoft/yii2-bootstrap": "*",
      "yiisoft/yii2-swiftmailer": "*",
      "2amigos/yii2-chartjs-widget": "~2.0",
      "yiisoft/yii2-imagine": "*"
  },

composer update