MacOS Mojave 系统之后，如果想安装 php5.6 版本的时候，无法用brew install php5.6 安装，因为在新的 brew 中已经废弃了 php5.6 和 php7.0，如果使用 brew search php 搜索出来的Php版本最低是 php@7.1 的，所以有相关需求的可以按照下面方法安装

添加源
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brew tap exolnet/homebrew-deprecated
搜索PHP
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brew search php
安装PHP
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brew install php@5.6
安装完后会提示如下信息：

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==> php@5.6
To enable PHP in Apache add the following to httpd.conf and restart Apache:
LoadModule php5_module /usr/local/opt/php@5.6/lib/httpd/modules/libphp5.so
<FilesMatch .php$>
SetHandler application/x-httpd-php

Finally, check DirectoryIndex includes index.php
DirectoryIndex index.php index.html
The php.ini and php-fpm.ini file can be found in:
/usr/local/etc/php/5.6/
php@5.6 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.
If you need to have php@5.6 first in your PATH run:
echo 'export PATH="/usr/local/opt/php@5.6/bin:$PATH"' >> ~/.bash_profile
echo 'export PATH="/usr/local/opt/php@5.6/sbin:$PATH"' >> ~/.bash_profile
For compilers to find php@5.6 you may need to set:
export LDFLAGS="-L/usr/local/opt/php@5.6/lib"
export CPPFLAGS="-I/usr/local/opt/php@5.6/include"

To have launchd start exolnet/deprecated/php@5.6 now and restart at login:
brew services start exolnet/deprecated/php@5.6
Or, if you don't want/need a background service you can just run:
sudo php-fpm
配置环境变量
vim ~/.bash_profile

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php56

export PATH=/usr/local/Cellar/php56/5.6.40/bin:$PATH

php56-fpm

export PATH=/usr/local/Cellar/php56/5.6.40/sbin:$PATH