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。
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