Composer簡介
Composer 是 PHP 的一個依賴管理工具。它允許你申明項目所依賴的代碼庫，它會在你的項目中為你安裝他們。Composer 不是一個包管理器。是的，它涉及 "packages" 和 "libraries"，但它在每個項目的基礎上進行管理，在你項目的某個目錄中（例如 vendor）進行安裝。默認情況下它不會在全局安裝任何東西。因此，這僅僅是一個依賴管理。
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：
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下應該也是可以運行的，composer可以設置忽略版本匹配，命令是：
composer install --ignore-platform-reqs
or
composer update --ignore-platform-reqs
再次執行composer命令可以正常安裝包了。
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
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為國內鏡像，不然下載速度慢的要死，執行：
composer config -g repo.packagist composer https://packagist.phpcomposer.com
OK，大功告成。
