composer 是php中用来管理依赖(dependency)关系的工具。你可以在自己的项目中声明所依赖的外部工具库(libraries)，composer会帮你安装这些依赖的库文件。
PHP Composer 安装
系统需求：
Composer 需要PHP5.3.2+ 以上的环境来运行。有几个敏感的PHP设置和编译标志也是必需的，但安装程序会发出警告当存在任何不兼容的情况。
比如PHP的扩展的要求是，安装或重新编译php without –disable-phar
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为了从源地址安装软件包，而不是简单的压缩文件包，您将需要安装软件包的版本控制工具，比如git、svn或hg等。
Composer 是兼容多平台的，其运行适用于Windows，Linux和OSX。
安装失败的错误消息：
curl -sS https://getcomposer.org/installer | PHP #!/usr/bin/env php some settings on your machine make Composer unable to work properly. Make sure that you fix the issues listed below and run this script again: The phar extension is missing. Install it or recompile php without –disable-phar Your PHP (5.2.9) is too old, you must upgrade to PHP 5.3.2 or higher.
我的本地默认开发环境是PHP 5.2.9，当我尝试在PHP 5.2.9上安装时，提示版本过低，需要升级到PHP 5.3.2 或者更高的版本。
于是我下载XAMPP 1.7.7 usb lite 版本。
E:\USB\Dropbox\phpstorm\php53>E:\USB\xampp_177\php\php.exe -v PHP 5.3.8 (cli) (built: Aug 23 2011 11:50:20) Copyright (c) 1997-2011 The PHP Group Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies
XAMPP 1.7.7 下的PHP环境是5.3.8。确认我的PHP环境大于等于PHP5.3.2后。
PHP 5.3 安装 Composer
E:\USB\Dropbox\phpstorm\php53>curl -sS https://getcomposer.org/installer | E:\USB\xampp_177\php\php.exe #!/usr/bin/env php Some settings on your machine may cause stability issues with Composer. If you encounter issues, try to change the following: The openssl extension is missing, which will reduce the security and stability o f Composer. If possible you should enable it or recompile php with –with-openssl Downloading… Composer successfully installed to: E:\USB\Dropbox\phpstorm\php53\composer.phar Use it: php composer.phar
注意：
我关闭了XAMPP 1.7.7 Apache的SSL模块，所以Composer提示我警告，为了安全因素建议我开启SSL模块。不开启也不影响使用。
安装时的参数 | PHP，一定要更改为真实的PHP环境路径，比如 | E:\USB\xampp_177\php\php.exe
使用Composer为PHP路径 + 命令，在我们安装的环境中，使用方法为：E:\USB\xampp_177\php/php.exe composer.phar 命令。
我安装的环境是在Windows下，如果在Linux下，而PHP 5.3 环境也不在默认PHP命令下时，应该采用 | /usr/php/53/usr/bin/php，例如：
curl -sS https://getcomposer.org/installer | /usr/php/53/usr/bin/php
安装完成后，检查安装版本。
E:\USB\Dropbox\phpstorm\php53>E:\USB\xampp_177\php/php.exe composer.phar -V Composer version 0209bd31a0ac3aeb2a68fc81e2d03c71072bef33
检查PHP Composer命令：
E:\USB\Dropbox\phpstorm\php53>E:\USB\xampp_177\php/php.exe composer.phar
______
/ ____/___ ____ ___ ____ ____ ________ _____
/ / / __ \/ __ `__ \/ __ \/ __ \/ ___/ _ \/ ___/
/ /___/ /_/ / / / / / / /_/ / /_/ (__ ) __/ /
\____/\____/_/ /_/ /_/ .___/\____/____/\___/_/
/_/
Composer version 0209bd31a0ac3aeb2a68fc81e2d03c71072bef33
Usage:
[options] command [arguments]
Options:
–help -h Display this help message.
–quiet -q Do not output any message.
–verbose -v Increase verbosity of messages.
–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
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
help Displays help for a command
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.
list Lists commands
require Adds required packages to your composer.json and installs the
m
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
这样我们在PHP5.3环境下的Composer已经安装完成。
类似的方法我们也可以安装PHP 5.4的环境上。
PHP 5.4 安装 Composer
下载XAMPP 1.8.1 usb lite 版本，将得到PHP5.4.7环境。
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E:\USB\Dropbox\phpstorm\php53>E:\USB\xampp_181\php\php.exe -v PHP 5.4.7 (cli) (built: Sep 12 2012 23:48:31) Copyright (c) 1997-2012 The PHP Group Zend Engine v2.4.0, Copyright (c) 1998-2012 Zend Technologies
确定版本高于PHP 5.3.2 后，开始安装：
E:\USB\Dropbox\phpstorm\php54>curl -sS https://getcomposer.org/installer | E:\USB\xampp_181\php\php.exe #!/usr/bin/env php Some settings on your machine may cause stability issues with Composer. If you encounter issues, try to change the following: The openssl extension is missing, which will reduce the security and stability o f Composer. If possible you should enable it or recompile php with –with-openssl Downloading… Composer successfully installed to: E:\USB\Dropbox\phpstorm\php54\composer.phar Use it: php composer.phar
安装完成后，检查安装版本。
E:\USB\Dropbox\phpstorm\php54>E:\USB\xampp_181\php/php.exe composer.phar -V Composer version 0209bd31a0ac3aeb2a68fc81e2d03c71072bef33
这样，PHP 5.4 环境下的Composer也安装成功了。
PHP Composer 使用
这里我只简单演示使用PHP Composer安装Symfony框架。
首先大概讲解一下命令的标准格式：
php composer.phar create-project symfony/framework-standard-edition path/ 2.2.1
第一个参数PHP为本地PHP开发环境，指向PHP运行命令。
第二个参数，为composer.phar方式下载。
第三个参数，为动作命令，例如create-project，目的是为了创建一个新的项目从一个软件包到指定目录。
第四个参数，为本地安装的目录。
第五个参数，为安装软件包的版本。
根据上述我们在PHP5.4环境下安装的Composer，我们的安装命令为：
E:\USB\Dropbox\phpstorm\php54>E:\USB\xampp_181\php/php.exe composer.phar create-project symfony/framework-standard-edition E:\USB\xampp_181\htdocs\Symfony 2.2.1
安装结果：
Installing symfony/framework-standard-edition (v2.2.1) – Installing symfony/framework-standard-edition (v2.2.1) Downloading: 100% Created project in E:\USB\xampp_181\htdocs\Symfony Loading composer repositories with package information Installing dependencies from lock file – Installing doctrine/lexer (v1.0) Downloading: 100% – Installing doctrine/annotations (v1.1) Downloading: 100% – Installing doctrine/cache (v1.0) Downloading: 100% – Installing doctrine/collections (v1.1) Downloading: 100% – Installing twig/twig (v1.12.2) Downloading: 100% – Installing psr/log (1.0.0) Downloading: 100% – Installing doctrine/inflector (v1.0) Downloading: 100% – Installing doctrine/common (2.4.0-RC1) Downloading: 100% – Installing symfony/symfony (v2.2.1) Downloading: 100% – Installing jdorn/sql-formatter (v1.2.0) Downloading: 100% – Installing doctrine/dbal (2.3.3) Downloading: 100% – Installing doctrine/doctrine-bundle (v1.2.0-beta1) Downloading: 100% – Installing doctrine/orm (2.3.3) Downloading: 100% – Installing jms/cg (1.0.0) Downloading: 100% – Installing phpoption/phpoption (1.2.0) Downloading: 100% – Installing jms/parser-lib (1.0.0) Downloading: 100% – Installing jms/metadata (1.3.0) Downloading: 100% – Installing jms/aop-bundle (1.0.0) Downloading: 100% – Installing jms/di-extra-bundle (1.3.0) Downloading: 100% – Installing jms/security-extra-bundle (1.4.0) Downloading: 100% – Installing sensio/distribution-bundle (v2.2.1) Downloading: 100% – Installing sensio/framework-extra-bundle (v2.2.1) Downloading: 100% – Installing sensio/generator-bundle (v2.2.1) Downloading: 100% – Installing kriswallsmith/assetic (v1.1.0-alpha4) Downloading: 100% – Installing symfony/assetic-bundle (v2.1.2) Downloading: 100% – Installing monolog/monolog (1.4.1) Downloading: 100% – Installing symfony/monolog-bundle (v2.2.0) Downloading: 100% – Installing swiftmailer/swiftmailer (v4.3.0) Downloading: 100% – Installing symfony/swiftmailer-bundle (v2.2.0) Downloading: 100% – Installing twig/extensions (v1.0.0) Downloading: 100% kriswallsmith/assetic suggests installing leafo/lessphp (Assetic provides the integration with the lessphp LESS compiler) kriswallsmith/assetic suggests installing leafo/scssphp (Assetic provides the integration with the scssphp SCSS compiler) kriswallsmith/assetic suggests installing leafo/scssphp-compass (Assetic provides the integration with the SCSS compass plugin) kriswallsmith/assetic suggests installing ptachoire/cssembed (Assetic provides the integration with phpcssembed to embed data uris) monolog/monolog suggests installing doctrine/couchdb (Allow sending log messages to a CouchDB server) monolog/monolog suggests installing ext-amqp (Allow sending log messages to an AMQP server (1.0+ required)) monolog/monolog suggests installing ext-mongo (Allow sending log messages to a MongoDB server) monolog/monolog suggests installing mlehner/gelf-php (Allow sending log messages to a GrayLog2 server) monolog/monolog suggests installing raven/raven (Allow sending log messages to a Sentry server) Generating autoload files Clearing the cache for the dev environment with debug true Installing assets using the hard copy option Installing assets for Symfony\Bundle\FrameworkBundle into web/bundles/framework Installing assets for Acme\DemoBundle into web/bundles/acmedemo Installing assets for Sensio\Bundle\DistributionBundle into web/bundles/sensiodistribution
从命令行结果，我们看到了整个Symfony框架的安装过程与结果。
PHP Composer 定义依赖关系
Composer在使用时会自动寻找composer.json这个文件。 composer.json将用来定义整个项目的依赖包。例如：
{
"require": {
"monolog/monolog": "1.2.*"
}
}
依赖包简单的说明，我们的项目需要一些从1.2开始的任何版本的monolog/monolog包。
Composer是PHP中的一个依赖关系管理工具。通过Composer我们可以在发布软件时节省大量的文件数目和控制依赖包的版本。
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