在composer require或者composer install命令中报错了，无法正常下载安装包，报错如下：

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpunit/php-code-coverage 4.0.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/php-code-coverage 4.0.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/php-code-coverage 4.0.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - Installation request for phpunit/php-code-coverage (locked at 4.0.8) -> satisfiable by phpunit/php-code-coverage[4.0.8].

  To enable extensions, verify that they are enabled in your .ini files:

解决办法：

composer require jpush/jpush --ignore-platform-reqs