執行composer install遇到錯誤：Your requirements could not be resolved to an installable set of packages. 這是因爲不匹配composer.json要求的版本。
完整錯誤如下：
-
vagrant@homestead:/usr/share/nginx/html/laravel-blog$ sudocomposerinstall -
Loadingcomposerrepositorieswithpackage information -
Installingdependencies (includingrequire-dev) fromlockfile -
Yourrequirementscouldnot 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下應該也是可以運行的，composer可以設置忽略版本匹配，命令是：
composerinstall --ignore-platform-reqs
or
composerupdate --ignore-platform-reqs
再次執行composer命令可以正常安裝包了。
如果提示警告：
-
Cannotcreatecachedirectory /home/vagrant/.composer/cache/repo/https---packagist.org/, or directoryis not writable. Proceedingwithoutcache -
Cannotcreatecachedirectory /home/vagrant/.composer/cache/files/, or directoryis not writable. Proceedingwithoutcache
這是在虛擬機中執行composer，提示這個目錄沒有可寫權限，composer無法緩存下載的包，這樣就每次都得重新下載，把目錄改成可寫可讀即可。
sudo chmod -R 777 /home/vagrant/.composer/cache/files/
另外，在虛擬機中也設置composer爲國內鏡像，不然下載速度慢的要死，執行：
composerconfig -g repo.packagistcomposerhttps://packagist.phpcomposer.com
OK，大功告成。