如何在Ubuntu 16.04上安装PHP7.1YAML扩展而不是7.2

我正试图在Ubuntu上安装yaml扩展

我的服务器上有PHP7.1

ubuntu@www1:~$ php -v
PHP 7.1.15-1+ubuntu14.04.1+deb.sury.org+2 (cli) (built: Mar  6 2018 11:27:08) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.1.15-1+ubuntu14.04.1+deb.sury.org+2, Copyright 
    (c) 1999-2018, by Zend Technologies

我在跟踪另一个SO帖子，上面写着“跑步”

sudo apt install libyaml-dev php-dev php-pear

当我这样做时，安装程序部分地说：

The following extra packages will be installed:
  dh-php php7.2-cli php7.2-common php7.2-dev php7.2-json php7.2-opcache
  php7.2-readline pkg-php-tools xml2
Suggested packages:
  dh-make
The following NEW packages will be installed:
  dh-php php-dev php7.2-cli php7.2-common php7.2-dev php7.2-json
  php7.2-opcache php7.2-readline pkg-php-tools xml2
0 upgraded, 10 newly installed, 0 to remove and 85 not upgraded.
Need to get 3,073 kB of archives.
After this operation, 18.3 MB of additional disk space will be used.
Do you want to continue? [Y/n] n
Abort.

那么，为什么安装程序要在PHP7.1服务器上安装PHP7.2扩展呢

我中止了安装并来到这里询问：

如何强制我的服务器保持在PHP7.1并安装yaml扩展？

您尝试过吗

sudo apt-get install -y php7.1-yaml

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我决定升级到PHP7.2这能回答你的问题吗？