今天调试php的代码，提示需要PHP56，于是看了下php在github上的指导文档
Installation
Setup the homebrew/dupes tap which has dependencies we need:
$ brew tap homebrew/dupes
Setup the homebrew/versions tap which has dependencies we need:
$ brew tap homebrew/versions
Then, run the following in your command-line:
$ brew tap homebrew/homebrew-php
Usage
Note: For a list of available configuration options run:
$ brew options php56
Once the tap is installed, you can install php53, php54, php55, php56, php70, or any formulae you might need via:
$ brew install php56
按照上面的指导操作，出现如下的告警，导致tap失败 ，后继无法install成功：
$ brew tap homebrew/homebrew-php
Warning: Tap homebrew/php already tapped.
解决：先untap，然后重新tap：
$ brew untap homebrew/php
done!