如何降级 PHP 7.1？

Question

最近，我将 Ubuntu 从 18.04 LTS 升级到了 18.10。这也将我的 PHP 7.1 升级到了此版本：PHP 7.1.28-1+ubuntu18.04.1+deb.sury.org+3。现在，我必须降级到 PHP 7.1.27，因为 7.1.28 引入了一些重大更改，导致我的 Symfony 项目无法运行。

我一直在努力降级，但没有成功。似乎没有低于该版本的东西可用：

 $ apt-cache policy php7.1               
 php7.1:
   Installed: 7.1.28-1+ubuntu18.04.1+deb.sury.org+3
   Candidate: 7.1.29-1+ubuntu18.10.1+deb.sury.org+1
   Version table:
      7.1.29-1+ubuntu18.10.1+deb.sury.org+1 500
         500 http://ppa.launchpad.net/ondrej/php/ubuntu cosmic/main amd64 Packages
         500 http://ppa.launchpad.net/ondrej/php/ubuntu cosmic/main i386 Packages
  *** 7.1.28-1+ubuntu18.04.1+deb.sury.org+3 100
         100 /var/lib/dpkg/status


 $ apt-cache madison php7.1
     php7.1 | 7.1.29-1+ubuntu18.10.1+deb.sury.org+1 | http://ppa.launchpad.net/ondrej/php/ubuntu cosmic/main amd64 Packages
     php7.1 | 7.1.29-1+ubuntu18.10.1+deb.sury.org+1 | http://ppa.launchpad.net/ondrej/php/ubuntu cosmic/main i386 Packages


$ apt-cache showpkg php7.1                                    
Package: php7.1
Versions: 
7.1.29-1+ubuntu18.10.1+deb.sury.org+1 (/var/lib/apt/lists/ppa.launchpad.net_ondrej_php_ubuntu_dists_cosmic_main_binary-amd64_Packages) (/var/lib/apt/lists/ppa.launchpad.net_ondrej_php_ubuntu_dists_cosmic_main_binary-i386_Packages)
 Description Language: 
             File: /var/lib/apt/lists/ppa.launchpad.net_ondrej_php_ubuntu_dists_cosmic_main_binary-amd64_Packages
              MD5: 86488d7a405bdcbac9ece7c307f30abb
 Description Language: en
             File: /var/lib/apt/lists/ppa.launchpad.net_ondrej_php_ubuntu_dists_cosmic_main_i18n_Translation-en
              MD5: 86488d7a405bdcbac9ece7c307f30abb
 Description Language: 
             File: /var/lib/dpkg/status
              MD5: 86488d7a405bdcbac9ece7c307f30abb

7.1.28-1+ubuntu18.04.1+deb.sury.org+3 (/var/lib/dpkg/status)
 Description Language: 
             File: /var/lib/apt/lists/ppa.launchpad.net_ondrej_php_ubuntu_dists_cosmic_main_binary-amd64_Packages
              MD5: 86488d7a405bdcbac9ece7c307f30abb
 Description Language: en
             File: /var/lib/apt/lists/ppa.launchpad.net_ondrej_php_ubuntu_dists_cosmic_main_i18n_Translation-en
              MD5: 86488d7a405bdcbac9ece7c307f30abb
 Description Language: 
             File: /var/lib/dpkg/status
              MD5: 86488d7a405bdcbac9ece7c307f30abb


Reverse Depends: 
Dependencies: 
7.1.29-1+ubuntu18.10.1+deb.sury.org+1 - libapache2-mod-php7.1 (16 (null)) php7.1-fpm (16 (null)) php7.1-cgi (0 (null)) php7.1-common (0 (null)) 
7.1.28-1+ubuntu18.04.1+deb.sury.org+3 - libapache2-mod-php7.1 (16 (null)) php7.1-fpm (16 (null)) php7.1-cgi (0 (null)) php7.1-common (0 (null)) 
Provides: 
7.1.29-1+ubuntu18.10.1+deb.sury.org+1 - php (= ) 
7.1.28-1+ubuntu18.04.1+deb.sury.org+3 - php (= ) 
Reverse Provides:

那么我该如何降级呢？不必严格按照 7.1.27 版本，只要低于 7.1.28 版本就可以。