composerをインストールしようとするとエラーでます。どのように解消すればよろしいでしょうか？

composerをインストールしようとすると以下のようなエラーがでます。
どのように解消すればいいでしょうか？

https://book.cakephp.org/3.0/ja/quickstart.html
こちらのクイックスタートガイドを実行している時にエラーが発生しました。

エラーはこちらになります。

Installing cakephp/app (3.5.1)

Installing cakephp/app (3.5.1): Downloading (100%)

Created project in cms
Loading composer repositories with package information
Updating dependencies (including require-dev)

The "http://packagist.org/p/symfony/debug%24a69a873e3505355c32922281b0e257d8216b391968be9d9832ced9ad2d8e29d0.json" file could not be downloaded: failed to open stream: Operation timed out
Retrying with degraded mode, check https://getcomposer.org/doc/articles/troubleshooting.md#degraded-mode for more info

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

Problem 1
- cakephp/cakephp 3.5.9 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.8 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.7 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.6 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.5 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.4 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.3 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.2 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.12 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.11 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.10 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.1 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- cakephp/cakephp 3.5.0 requires ext-intl * -> the requested PHP extension intl is missing from your system.
- Installation request for cakephp/cakephp 3.5.* -> satisfiable by cakephp/cakephp[3.5.0, 3.5.1, 3.5.10, 3.5.11, 3.5.12, 3.5.2, 3.5.3, 3.5.4, 3.5.5, 3.5.6, 3.5.7, 3.5.8, 3.5.9].

To enable extensions, verify that they are enabled in your .ini files:
- /usr/local/etc/php/5.6/php.ini
You can also run php --ini inside terminal to see which files are used by PHP in CLI mode.