執行composer install遇到錯誤：Your requirements could not be resolved to an installable set of packages. 這是因為不匹配composer.json要求的版本。
完整錯誤如下：
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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].
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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：
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composerinstall --ignore-platform-reqs
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或者
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composerupdate --ignore-platform-reqs
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再次執行composer命令可以正常安裝包了。
如果提示警告：
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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
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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。
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sudo
chmod
-R 777 /home/vagrant/.composer/cache/files/
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另外，在虛擬機中也設置composer為國內鏡像，不然下載速度慢的要死，執行：
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composerconfig -g repo.packagistcomposerhttps:
//packagist.phpcomposer.com
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