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：
PHP
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。
