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Ubuntu 上官方的源，比如 Ubuntu14.04 默认源中的是 PHP5.6.x、Ubuntu16.04 默认源中的是 PHP7.0.x，那么如果想在 Ubuntu16.04 上安装 PHP7.1，PHP7.2，应该怎么办呢？
答案是通过第三方的源来安装，ppa:ondrej/php 是一个比较知名的 PHP 源（目前维护 php5.6,php7.0,php7.1,php7.2）。下面就用这个源来安装自己想要的 PHP 版本。
1. 通过源安装 PHP
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install php7.1 # for PHP 7.1
sudo apt-get install php7.0 # for PHP 7.0
sudo apt-get install php5.6 # for PHP 5.6
2. 安装相关版本的 PHP 模块
sudo apt-cache search php7*
…
php-all-dev – package depending on all supported PHP development packages
php7.0 – server-side, HTML-embedded scripting language (metapackage)
php7.0-cgi – server-side, HTML-embedded scripting language (CGI binary)
php7.0-cli – command-line interpreter for the PHP scripting language
php7.0-common – documentation, examples and common module for PHP
php7.0-curl – CURL module for PHP
php7.0-dev – Files for PHP7.0 module development
php7.0-gd – GD module for PHP
php7.0-gmp – GMP module for PHP
…
…
…
php7.1-xml – DOM, SimpleXML, WDDX, XML, and XSL module for PHP
php7.1-xmlrpc – XMLRPC-EPI module for PHP
php7.1-zip – Zip module for PHP
php7.1-opcache – Zend OpCache module for PHP
php7.1 – server-side, HTML-embedded scripting language (metapackage)
php7.1-xsl – XSL module for PHP (dummy)
…
…
php7.2-bcmath – Bcmath module for PHP
php7.2-bz2 – bzip2 module for PHP
php7.2-cgi – server-side, HTML-embedded scripting language (CGI binary)
php7.2-cli – command-line interpreter for the PHP scripting language
php7.2-common – documentation, examples and common module for PHP
php7.2-curl – CURL module for PHP
php7.2-dba – DBA module for PHP
3. 选择需要的模块，安装之
sudo apt-get install php7.2-curl php7.2-dev php7.2-gd php7.2-imap php7.2-intl php7.2-mbstring php7.2-mysql php7.2-xml php7.2-zip
4. 切换 PHP 版本
sudo update-alternatives –config php
5. 设置 Apache 与正确的 PHP 版本一起运行
sudo a2dismod php7.1 # unload the current version
sudo a2enmod php5.6 # load the version you need
sudo service apache2 restart # restart webserver to apply
到此结束，再也不用担心安装 PHP，切换 PHP 版本这些事情啦。
参考：
- https://askubuntu.com/questions/109404/how-do-i-install-different-upgrade-or-downgrade-php-version-in-still-supported
- https://tecadmin.net/install-php-7-on-ubuntu/
