include/require
如果没有提供路径，则会查找include_path，如果没有找到，则会查找当前脚本所在目录和当前工作目录。如果均失败的话，include 会产生一个 warnning，require会产生一个 fatal error.
当一个文件被 included时，被包含文件中的代码继承了include所在行的变量作用域。任何在include处可见的变量，对于被包含的代码也可见（相当于将被包含的代码复制并替换include命令。）但是，被包含文件中定义的函数和类都拥有全局作用域。
在解析被包含的文件时，解析器会切换为 HTML模式，所以任何没有包含在 php tag中的文本都会当做HTML处理，也就是会被原样 echo 给客户端。
include失败会返回FALSE，并产生一个warning。如果include成功，并且被包含文件中没有return语句，include会返回1。 如果被包含文件中有return语句，那么执行到return语句时，会停止对被包含文件的处理，并返回return提供的值，就像调用一个普通的函数一样。
如果被包含文件中定义了函数，并且被包含了两次，则会造成 fatal error，因为函数已经被声明过了。最好使用 include_once。
安装
sudo apt-add-repository ppa:ondrej/php5php-xxx都是PHP的扩展包，比如php-gd，php-mysql，php-xdebug需要手动安装- Apache 通过
libapache2-mod-php7.1模块与PHP互动，配置文件为/etc/apache2/mod-enabled/php7.0/中 /usr/bin/php也只是一个alternative，可以使用update-alternative改变当前php命令使用的版本
安装过程中的处理：
Setting up php7.1-common (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/calendar.ini with new version
Creating config file /etc/php/7.1/mods-available/ctype.ini with new version
Creating config file /etc/php/7.1/mods-available/exif.ini with new version
Creating config file /etc/php/7.1/mods-available/fileinfo.ini with new version
Creating config file /etc/php/7.1/mods-available/ftp.ini with new version
Creating config file /etc/php/7.1/mods-available/gettext.ini with new version
Creating config file /etc/php/7.1/mods-available/iconv.ini with new version
Creating config file /etc/php/7.1/mods-available/pdo.ini with new version
Creating config file /etc/php/7.1/mods-available/phar.ini with new version
Creating config file /etc/php/7.1/mods-available/posix.ini with new version
Creating config file /etc/php/7.1/mods-available/shmop.ini with new version
Creating config file /etc/php/7.1/mods-available/sockets.ini with new version
Creating config file /etc/php/7.1/mods-available/sysvmsg.ini with new version
Creating config file /etc/php/7.1/mods-available/sysvsem.ini with new version
Creating config file /etc/php/7.1/mods-available/sysvshm.ini with new version
Creating config file /etc/php/7.1/mods-available/tokenizer.ini with new version
Setting up php7.1-json (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/json.ini with new version
Setting up php7.1-opcache (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/opcache.ini with new version
Setting up php7.1-readline (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/readline.ini with new version
Setting up php7.1-cli (7.1.0-5+deb.sury.org~xenial+1) ...
update-alternatives: using /usr/bin/php7.1 to provide /usr/bin/php (php) in auto mode
update-alternatives: using /usr/bin/phar7.1 to provide /usr/bin/phar (phar) in auto mode
update-alternatives: using /usr/bin/phar.phar7.1 to provide /usr/bin/phar.phar (phar.phar) in auto mode
Creating config file /etc/php/7.1/cli/php.ini with new version
Setting up libapache2-mod-php7.1 (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/apache2/php.ini with new version
libapache2-mod-php7.1: php7.0 module already enabled, not enabling PHP 7.1
Setting up libapache2-mod-php (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php7.0-common (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-mcrypt (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-mbstring (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-xml (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-readline (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-opcache (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-mysql (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-json (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up libwebp5:amd64 (0.4.4-1+deb.sury.org~xenial+1) ...
Setting up libgd3:amd64 (2.2.3-3+deb.sury.org~xenial+0) ...
Setting up php7.0-gd (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-curl (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up php7.0-cli (7.0.14-2+deb.sury.org~xenial+1) ...
Replacing config file /etc/php/7.0/cli/php.ini with new version
Setting up libapache2-mod-php7.0 (7.0.14-2+deb.sury.org~xenial+1) ...
Installing new version of config file /etc/apache2/mods-available/php7.0.load ...
libapache2-mod-php7.0: not switching MPM - already enabled
Setting up php7.0 (7.0.14-2+deb.sury.org~xenial+1) ...
Setting up libjbig0:amd64 (2.1-3.1+deb.sury.org~xenial+1) ...
Setting up libpcre16-3:amd64 (2:8.39-1+deb.sury.org~xenial+1) ...
Setting up libsodium18:amd64 (1.0.10-1+deb.sury.org~xenial+1) ...
Setting up libssl-doc (1.0.2j-1+deb.sury.org~xenial+1) ...
Setting up libssl-dev:amd64 (1.0.2j-1+deb.sury.org~xenial+1) ...
Setting up libwebpmux1:amd64 (0.4.4-1+deb.sury.org~xenial+1) ...
Setting up libzmq5:amd64 (4.2.0-1+deb.sury.org~xenial+1) ...
Setting up openssl (1.0.2j-1+deb.sury.org~xenial+1) ...
Setting up php7.1-curl (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/curl.ini with new version
Setting up php-curl (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php7.1-gd (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/gd.ini with new version
Setting up php-gd (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php7.1-xml (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/dom.ini with new version
Creating config file /etc/php/7.1/mods-available/simplexml.ini with new version
Creating config file /etc/php/7.1/mods-available/wddx.ini with new version
Creating config file /etc/php/7.1/mods-available/xml.ini with new version
Creating config file /etc/php/7.1/mods-available/xmlreader.ini with new version
Creating config file /etc/php/7.1/mods-available/xmlwriter.ini with new version
Creating config file /etc/php/7.1/mods-available/xsl.ini with new version
Setting up php-xml (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php-pear (1:1.10.1+submodules+notgz-8+donate.sury.org~xenial+2) ...
Setting up php-gettext (1.0.11-2+deb.sury.org~xenial+1) ...
Setting up php7.1-mbstring (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/mbstring.ini with new version
Setting up php-mbstring (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php7.1-mcrypt (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/mcrypt.ini with new version
Setting up php-mcrypt (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php7.1-mysql (7.1.0-5+deb.sury.org~xenial+1) ...
Creating config file /etc/php/7.1/mods-available/mysqlnd.ini with new version
Creating config file /etc/php/7.1/mods-available/mysqli.ini with new version
Creating config file /etc/php/7.1/mods-available/pdo_mysql.ini with new version
Setting up php-mysql (1:7.1+49+deb.sury.org~xenial+4) ...
Setting up php-xdebug (2.5.0-1+deb.sury.org~xenial+1) ...
Processing triggers for libc-bin (2.23-0ubuntu5) ...
Processing triggers for libapache2-mod-php7.1 (7.1.0-5+deb.sury.org~xenial+1) ...
- 问题，现在通过
a2enmod php7.1启用了PHP7.1，那么对于PHP的请求，会交给PHP7.0呢？还是PHP7.1呢？ =》 默认的还是PHP7.0，需要自己手动a2dismod php7.0才行 - php-pear是什么？