Depedencias en php con Composer, Composer basado en nodejs,

composer.txt

Instalacion de forma global de composer en Linux
 $ curl -sS https://getcomposer.org/installer | php
 $ sudo mv composer.phar /usr/bin/composer
 $ composer -V

Instalacion del proyecto de forma local, via composer
 $ curl -sS https://getcomposer.org/installer | php
 $ php composer.phar create-project laravel/laravel my-project.com --prefer-dist
  
Empezamos un proyecto con composer:
$ composer init

Actualizamos una libreria o todas las librerias que descarge:
 $ composer update [vendor/package]

Para instalar librerias, ejemplos:  
   composer require vendor/package:~X.Y
 $ composer require 'symfony/symfony:2.8.0'
 $ composer require monolog/monolog
 $ composer require laralibs/modular:dev-master
 $ composer require 'phpunit/phpunit=4.0.*'
 $ composer require laralib/l5scaffold:dev-master
 
 $ composer require "pingpong/modules:~2.0"
 {
    "require": {
        "pingpong/modules": "~2.0"
    }
 }
 
Error / Warnings
  PHP  Warning:  proc_open(): fork failed - Cannot allocate memory in 
  phar:///usr/local/bin/composer/vendor/symfony/console/Application.php on line 959
  Warning: proc_open(): fork failed - Cannot allocate memory in 
  phar:///usr/local/bin/composer/vendor/symfony/console/Application.php on line 959
  [ErrorException]
  proc_open(): fork failed - Cannot allocate memory
 
  This could be happening because the VPS runs out of memory and has no Swap space enabled.
  $ free -m
  total used free shared buffers cached
  Mem: 2048 357 1690 0 0 237
  -/+ buffers/cache: 119 1928
  Swap: 0 0 0
  Solve:
  To enable the swap you can use for example:
  $ sudo /bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024
  $ sudo /sbin/mkswap /var/swap.1
  $ sudo /sbin/swapon /var/swap.1
  $ sudo /bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024 && sudo /sbin/mkswap /var/swap.1 && sudo /sbin/swapon /var/swap.1
  

Descargamos la librerias que ya confgure: 
$ composer install
Lee el archivo composer.json para recoger la lista de dependencias
Lee el archivo composer.lock para ver la version instalada por el commiter y descarga las dependecias con la version indicada.
Si una dependencia no esta en el archivo .lock pero si en el .json composer la descargara y la registrara en .lock