Php 试图在项目上安装composer，但出现需求错误消息

按照此链接的说明刚刚安装了composer：

进入ProjectRepo运行

composer安装

，但它产生了

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

Problem 1
    - Installation request for phpunit/php-code-coverage 4.0.8 -> satisfiable by phpunit/php-code-coverage[4.0.8].
    - phpunit/php-code-coverage 4.0.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
  Problem 2
    - Installation request for phpunit/phpunit 5.7.20 -> satisfiable by phpunit/phpunit[5.7.20].
    - phpunit/phpunit 5.7.20 requires ext-dom * -> the requested PHP extension dom is missing from your system.

  To enable extensions, verify that they are enabled in your .ini files:
- /etc/php/7.2/cli/php.ini
- /etc/php/7.2/cli/conf.d/10-mysqlnd.ini
- /etc/php/7.2/cli/conf.d/10-opcache.ini
- /etc/php/7.2/cli/conf.d/10-pdo.ini
- /etc/php/7.2/cli/conf.d/20-calendar.ini
- /etc/php/7.2/cli/conf.d/20-ctype.ini
- /etc/php/7.2/cli/conf.d/20-exif.ini
- /etc/php/7.2/cli/conf.d/20-fileinfo.ini
- /etc/php/7.2/cli/conf.d/20-ftp.ini
- /etc/php/7.2/cli/conf.d/20-gettext.ini
- /etc/php/7.2/cli/conf.d/20-iconv.ini
- /etc/php/7.2/cli/conf.d/20-json.ini
- /etc/php/7.2/cli/conf.d/20-mbstring.ini
- /etc/php/7.2/cli/conf.d/20-mysqli.ini
- /etc/php/7.2/cli/conf.d/20-pdo_mysql.ini
- /etc/php/7.2/cli/conf.d/20-phar.ini
- /etc/php/7.2/cli/conf.d/20-posix.ini
- /etc/php/7.2/cli/conf.d/20-readline.ini
- /etc/php/7.2/cli/conf.d/20-shmop.ini
- /etc/php/7.2/cli/conf.d/20-sockets.ini
- /etc/php/7.2/cli/conf.d/20-sysvmsg.ini
- /etc/php/7.2/cli/conf.d/20-sysvsem.ini
- /etc/php/7.2/cli/conf.d/20-sysvshm.ini
- /etc/php/7.2/cli/conf.d/20-tokenizer.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

我是Ubuntu新手，以前在Windows中从未遇到过这个问题。 这是通过Ubuntu安装composer的常见步骤吗？

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如何解决此问题？

如果您使用digitalocean作为主机，我的建议是与他们联系。我认为他们也有现场支持

这是因为您正在使用的计划可能有一些限制或需要权限

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如果您正在电脑中安装，请参考一些专门用于教程的博客/网站，而不是关注任何特定的设置

这在每次运行composer时都很常见：它检查扩展。阅读错误消息会给你一个明显的提示，告诉你应该复制什么