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Composer简介

Composer 是 PHP 的一个依赖管理工具。它允许你申明项目所依赖的代码库，它会在你的项目中为你安装他们。Composer 不是一个包管理器。是的，它涉及 "packages" 和 "libraries"，但它在每个项目的基础上进行管理，在你项目的某个目录中（例如 vendor）进行安装。默认情况下它不会在全局安装任何东西。因此，这仅仅是一个依赖管理。

执行composer install遇到错误：Your requirements could not be resolved to an installable set of packages. 这是因为不匹配composer.json要求的版本。

完整错误如下：

vagrant@homestead:/usr/share/nginx/html/laravel-blog$ sudo composer install

Loading composer repositories with package information

Installing dependencies (including require-dev) from lock file

Your requirements could not be resolved to an installable set of packages.

Problem 1

- Installation request for doctrine/instantiator 1.0.3 -> satisfiable by doctrine/instantiator[1.0.3].

- doctrine/instantiator 1.0.3 requires php ~5.3 -> your PHP version (7.0.3) does not satisfy that requirement.

Problem 2

- doctrine/instantiator 1.0.3 requires php ~5.3 -> your PHP version (7.0.3) does not satisfy that requirement.

- phpunit/phpunit-mock-objects 2.3.0 requires doctrine/instantiator ~1.0,>=1.0.1 -> satisfiable by doctrine/instantiator[1.0.3].

- Installation request for phpunit/phpunit-mock-objects 2.3.0 -> satisfiable by phpunit/phpunit-mock-objects[2.3.0].

提示我的PHP 7版本太高，不符合composer.json需要的版本，但是在PHP 7下应该也是可以运行的，composer可以设置忽略版本匹配，命令是：

composer install --ignore-platform-reqs

or

composer update --ignore-platform-reqs

再次执行composer命令可以正常安装包了。

如果提示警告：

Cannot create cache directory /home/vagrant/.composer/cache/repo/https---packagist.org/, or directory is not writable. Proceeding without cache

Cannot create cache directory /home/vagrant/.composer/cache/files/, or directory is not writable. Proceeding without cache

这是在虚拟机中执行composer，提示这个目录没有可写权限，composer无法缓存下载的包，这样就每次都得重新下载，把目录改成可写可读即可。

sudo chmod -R 777 /home/vagrant/.composer/cache/files/

另外，在虚拟机中也设置composer为国内镜像，不然下载速度慢的要死，执行：

composer config -g repo.packagist composer https://packagist.phpcomposer.com

OK，大功告成。