1.首先安装brew
/usr/bin/ruby -e "$(curl -fsSLhttps://raw.githubusercontent.com/Homebrew/install/master/install)"
2.安装composer
brew install composer
安装的composer可能还不是最新版本的，可以升级一下
composer self-update
可能会报错。代码为：

Error: No available formula with the name "php56-redis" 
==> Searching for a previously deleted formula...
Error: No previously deleted formula found.
==> Searching for similarly named formulae...
Error: No similarly named formulae found.
==> Searching taps...
Error: No formulae found in taps.

3.在brew中添加tap

brew tap homebrew/homebrew-php

4.php56-redis 扩展

brew install php56-redis

就能安装成功了，此过程需要一些时间，大概十分钟左右
5.不要忘了重启apache

sudo apachectl restart

6.安装composer
brew install composer
截图：

7.验证composer

Mac安装composer爬过的坑