默认php版本为7.1，需要升级，并且设置成默认的最新版。
brew search php
查到最新版本之后，目前最新@7.3
brew install php@7.3
设置环境变量
vim ~/.bash_profile
环境变量加入一下代码：这个环境设置在php下载过程中有提示：
If you need to have php@7.3 first in your PATH run:
echo 'export PATH="/usr/local/opt/php@7.3/bin:$PATH"' >> /Users/yanruicheng/.bash_profile
echo 'export PATH="/usr/local/opt/php@7.3/sbin:$PATH"' >> /Users/yanruicheng/.bash_profile
For compilers to find php@7.3 you may need to set:
export LDFLAGS="-L/usr/local/opt/php@7.3/lib"
export CPPFLAGS="-I/usr/local/opt/php@7.3/include"
To have launchd start php@7.3 now and restart at login:
brew services start php@7.3
保存，重启变量设置：
source ~/.bash_profile
然后php -v 查看版本，已经显示是最新版了。