ubuntu16.04 系统源自带是7.0的，如何降级安装PHP 5.6呢?
apt-get install -y language-pack-en-base
LC_ALL=en_US.UTF-8 add-apt-repository ppa:ondrej/php
add-apt-repository ppa:ondrej/php
apt-get update
apt-cache search php5
apt-get install php5.6-common
apt-get install libapache2-mod-php5.6
如果需要解析xml apt-get install php5.6-xml
Warming：
apt-get install libapache2-mod-php5.6
libapache2-mod-php5.6: php7.0 module already enabled, not enabling PHP 5.6
apt-get install libapache2-mod-php5.6
php7.0 module already enabled, not enabling PHP 5.6update-alternatives --set php /usr/bin/php5.6
解决方法：
update-alternatives --set php /usr/bin/php5.6
php -v
PHP 5.6.29-1+deb.sury.org~xenial+1 (cli)
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