当我尝试安装CakePHP时，出现intl缺失的错误：
$ composer self-update && composer create-project --prefer-dist cakephp/app blog
You are already using composer version 1.5.2 (stable channel).
Installing cakephp/app (3.5.0)
- Installing cakephp/app (3.5.0): Loading from cache
Created project in blog
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
- 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 3.5.1 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, 3.5.2, 3.5.3, 3.5.4, 3.5.5].
当我尝试安装php56-intl时，我收到一条已安装的消息：
$ brew install php56-intl
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 2 taps (homebrew/core, homebrew/php).
==> New Formulae
bchunk dps8m e2tools heartbeat kaitai-struct-compiler simg2img
==> Updated Formulae
abcmidi debianutils getdns homebrew/php/php55-molten inspircd mdds ompl scalaenv terragrunt
asciidoc dssim geth homebrew/php/php56-molten itstool mikutter ortp scalastyle tintin
bazel erlang gofabric8 homebrew/php/php70-molten kerl mupdf pandoc scm-manager vapoursynth
bit exploitdb grafana homebrew/php/php71-molten libgsf mupdf-tools poco sjk wireguard-tools
cmake faas-cli grakn i2p libmaxminddb mypy poppler sourcekitten yara
convox firebase-cli gsmartcontrol imagemagick libpqxx nativefier prometheus swiftformat
cppad gegl homebrew/php/php54-molten imagemagick@6 libuv node-build resty swiftlint
==> Renamed Formulae
newsbeuter -> newsboat
Warning: homebrew/php/php56-intl 5.6.32_10 is already installed
为什么会这样？我该怎么做才能解决它？
编辑：我在Mac OS X上。正在运行which php会返回PHP 5.6.28 ...