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$ sudocomposerinstallloadingcomposerrepositorieswithpackage informationinstallingdependencies (includingrequire-dev) fromlockfileyourrequirementscouldnot beresolvedto aninstallablesetofpackages. problem 1 - installationrequestfor doctrine/instantiator 1.0.3 -> satisfiablebydoctrine/instantiator[1.0.3]. - doctrine/instantiator 1.0.3 requiresphp ~5.3 -> yourphpversion (7.0.3) doesnot satisfythatrequirement. problem 2 - doctrine/instantiator 1.0.3 requiresphp ~5.3 -> yourphpversion (7.0.3) doesnot satisfythatrequirement. - phpunit/phpunit-mock-objects 2.3.0 requiresdoctrine/instantiator ~1.0,>=1.0.1 -> satisfiablebydoctrine/instantiator[1.0.3]. - installationrequestfor phpunit/phpunit-mock-objects 2.3.0 -> satisfiablebyphpunit/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：
composerinstall --ignore-platform-reqs
or
composerupdate --ignore-platform-reqs
再次执行composer命令可以正常安装包了。
如果提示警告：
cannotcreatecachedirectory /home/vagrant/.composer/cache/repo/https---packagist.org/, or directoryis not writable. proceedingwithoutcachecannotcreatecachedirectory /home/vagrant/.composer/cache/files/, or directoryis not writable. proceedingwithoutcache
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。
sudochmod -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：
composerconfig -g repo.packagistcomposerhttps://packagist.phpcomposer.com
ok，大功告成。