composer 无法下载程序/包

问题描述：

composer 无法下载程序或依赖包，出现以下三种错误：

1. PHP Fatal error: Uncaught Error: Call to undefined function Composer\XdebugHandler\putenv() in phar:///usr/local/bin/composer/vendor/composer/xdebug-handler/src/Process.php:93
2. [Symfony\Component\Process\Exception\RuntimeException]
   The Process class relies on proc_open, which is not available on your PHP installation.
3. PHP Fatal error: Uncaught Error: Call to undefined function Symfony\Component\Process\proc_get_status() in phar:///usr/local/bin/composer/vendor/symfony/process/Process.php:1279

解决办法：

一些 PHP 函数在服务器中被禁止了，开放所需函数即可：putenv，proc_open，proc_get_status。