apt-get update & upgrade 后出现了以下错误，Magento 2 的站点也出现 500 错误。
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/dom.so' - /usr/lib/php/20151012/dom.so: undefined symbol: php_libxml_node_free_list in Unknown on line 0
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/xmlreader.so' - /usr/lib/php/20151012/xmlreader.so: undefined symbol: dom_node_class_entry in Unknown on line 0
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/xsl.so' - /usr/lib/php/20151012/xsl.so: undefined symbol: dom_node_class_entry in Unknown on line 0
解决办法，重新安装了一下
apt-get install -y php7.0 libapache2-mod-php7.0 php7.0 php7.0-common php7.0-gd php7.0-mysql php7.0-mcrypt php7.0-curl php7.0-intl php7.0-xsl php7.0-mbstring php7.0-zip php7.0-bcmath php7.0-iconv
servie apache2 restart 后正常了。
