Laravelプロジェクトにてcomposer updateができない

Question

##事象

• composer updateを行うと以下のエラーとなる

Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpunit/phpunit 6.5.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.14 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.5.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.4.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.4.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.4.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.4.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.4.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.3.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.3.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.2.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.2.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.2.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.2.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.2.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.1.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.1.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.1.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.1.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.1.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 6.0.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - Installation request for phpunit/phpunit ~6.0 -> satisfiable by phpunit/phpunit[6.0.0, 6.0.1, 6.0.10, 6.0.11, 6.0.12, 6.0.13, 6.0.2, 6.0.3, 6.0.4, 6.0.5, 6.0.6, 6.0.7, 6.0.8, 6.0.9, 6.1.0, 6.1.1, 6.1.2, 6.1.3, 6.1.4, 6.2.0, 6.2.1, 6.2.2, 6.2.3, 6.2.4, 6.3.0, 6.3.1, 6.4.0, 6.4.1, 6.4.2, 6.4.3, 6.4.4, 6.5.0, 6.5.1, 6.5.10, 6.5.11, 6.5.12, 6.5.13, 6.5.14, 6.5.2, 6.5.3, 6.5.4, 6.5.5, 6.5.6, 6.5.7, 6.5.8, 6.5.9].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php.ini
    - /etc/php.d/20-bz2.ini
    - /etc/php.d/20-calendar.ini
    - /etc/php.d/20-ctype.ini
    - /etc/php.d/20-curl.ini
    - /etc/php.d/20-exif.ini
    - /etc/php.d/20-fileinfo.ini
    - /etc/php.d/20-ftp.ini
    - /etc/php.d/20-gettext.ini
    - /etc/php.d/20-iconv.ini
    - /etc/php.d/20-json.ini
    - /etc/php.d/20-mbstring.ini
    - /etc/php.d/20-mysqlnd.ini
    - /etc/php.d/20-pdo.ini
    - /etc/php.d/20-phar.ini
    - /etc/php.d/20-sockets.ini
    - /etc/php.d/20-sqlite3.ini
    - /etc/php.d/20-tokenizer.ini
    - /etc/php.d/30-mysqli.ini
    - /etc/php.d/30-pdo_mysql.ini
    - /etc/php.d/30-pdo_sqlite.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

##解決したいこと

• composer updateを正常に完了したい

##環境

• AWS EC2
• PHPのバージョン:7.2.16
• Laravel 5.6

##備考

• 開発環境では正常にcomposer updateを実行できる

####開発環境

• OS:mac
• PHPのバージョン: 7.2.5
• Laravel 5.6

Answer 1

requested PHP extension dom is missing from your system

と書いてありますので、
PHP extension dom が足りていないよ、との事です。

PHP extension dom

は
php-xmlの事っぽいので、php-xmlを追加でインストールしてください。