title: Install PHP 7.4 met with "unmet dependencies", how to solve this?
body: <p>I know a lot of topics have the same question but I've tried everything and nothing works for me.
  So I'm trying to install php 7.4 on my ubuntu stretch. I've done the following commands I found on the Internet:</p>
  <pre><code>apt install ca-certificates apt-transport-https wget gnupg
  wget -q https://packages.sury.org/php/apt.gpg -O- | apt-key add -
  echo &quot;deb https://packages.sury.org/php/ buster main&quot; | tee /etc/apt/sources.list.d/php.list
  apt update
  apt install php7.4
  </code></pre>
  <p>And then I get the error:</p>
  <pre><code>Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  Some packages could not be installed. This may mean that you have
  requested an impossible situation or if you are using the unstable
  distribution that some required packages have not yet been created
  or been moved out of Incoming.
  The following information may help to resolve the situation:
  
  The following packages have unmet dependencies:
   php7.4 : Depends: libapache2-mod-php7.4 but it is not going to be installed or
                     php7.4-fpm but it is not going to be installed or
                     php7.4-cgi but it is not going to be installed
            Depends: php7.4-common but it is not going to be installed
  E: Unable to correct problems, you have held broken packages.
  </code></pre>
  <p>I saw that someone followed the error by trying to install the depedencies that were causing the problem. So I then did</p>
  <pre><code>apt install php7.4-common
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  Some packages could not be installed. This may mean that you have
  requested an impossible situation or if you are using the unstable
  distribution that some required packages have not yet been created
  or been moved out of Incoming.
  The following information may help to resolve the situation:
  
  The following packages have unmet dependencies:
   php7.4-common : Depends: libc6 (&gt;= 2.27) but 2.24-11+deb9u4 is to be installed
  E: Unable to correct problems, you have held broken packages.
  </code></pre>
  <p>I saw somewhere that it could be something in my /etc/apt/sources.list so here it is:</p>
  <pre><code># Generated by distrobuilder
  deb http://deb.debian.org/debian stretch main
  deb http://security.debian.org/debian-security stretch/updates main
  </code></pre>
  <p>I'm stuck, I don't see what's wrong. Could you help? Thanks</p>