title: Requirements could not be resolved
body: <p>I am on Linux Ubuntu and I have a Symfony 4 project and I did not succeed to make that functions. After updating my OS, I have a problem. When I launch for this project: </p>
  
  <p>composer install</p>
  
  <p>I got this message:</p>
  
  <pre><code>Loading composer repositories with package information
  Installing dependencies (including require-dev) from lock file
  Your requirements could not be resolved to an installable set of packages.
  
    Problem 1
      - Installation request for nexylan/slack v2.0.0 -&gt; satisfiable by nexylan/slack[v2.0.0].
      - nexylan/slack v2.0.0 requires ext-mbstring * -&gt; the requested PHP extension mbstring is missing from your system.
    Problem 2
      - Installation request for symfony/framework-bundle v4.0.14 -&gt; satisfiable by symfony/framework-bundle[v4.0.14].
      - symfony/framework-bundle v4.0.14 requires ext-xml * -&gt; the requested PHP extension xml is missing from your system.
    Problem 3
      - Installation request for symfony/debug-bundle v4.0.4 -&gt; satisfiable by symfony/debug-bundle[v4.0.4].
      - symfony/debug-bundle v4.0.4 requires ext-xml * -&gt; the requested PHP extension xml is missing from your system.
    Problem 4
      - symfony/framework-bundle v4.0.14 requires ext-xml * -&gt; the requested PHP extension xml is missing from your system.
      - symfony/maker-bundle v1.4.0 requires symfony/framework-bundle ^3.4|^4.0 -&gt; satisfiable by symfony/framework-bundle[v4.0.14].
      - Installation request for symfony/maker-bundle v1.4.0 -&gt; satisfiable by symfony/maker-bundle[v1.4.0].
  
    To enable extensions, verify that they are enabled in your .ini files:
      - /etc/php/7.4/cli/php.ini
      - /etc/php/7.4/cli/conf.d/10-opcache.ini
      - /etc/php/7.4/cli/conf.d/10-pdo.ini
      - /etc/php/7.4/cli/conf.d/20-apcu.ini
      - /etc/php/7.4/cli/conf.d/20-calendar.ini
      - /etc/php/7.4/cli/conf.d/20-ctype.ini
      - /etc/php/7.4/cli/conf.d/20-exif.ini
      - /etc/php/7.4/cli/conf.d/20-ffi.ini
      - /etc/php/7.4/cli/conf.d/20-fileinfo.ini
      - /etc/php/7.4/cli/conf.d/20-ftp.ini
      - /etc/php/7.4/cli/conf.d/20-gettext.ini
      - /etc/php/7.4/cli/conf.d/20-iconv.ini
      - /etc/php/7.4/cli/conf.d/20-imagick.ini
      - /etc/php/7.4/cli/conf.d/20-json.ini
      - /etc/php/7.4/cli/conf.d/20-phar.ini
      - /etc/php/7.4/cli/conf.d/20-posix.ini
      - /etc/php/7.4/cli/conf.d/20-readline.ini
      - /etc/php/7.4/cli/conf.d/20-shmop.ini
      - /etc/php/7.4/cli/conf.d/20-sockets.ini
      - /etc/php/7.4/cli/conf.d/20-sysvmsg.ini
      - /etc/php/7.4/cli/conf.d/20-sysvsem.ini
      - /etc/php/7.4/cli/conf.d/20-sysvshm.ini
      - /etc/php/7.4/cli/conf.d/20-tokenizer.ini
      - /etc/php/7.4/cli/conf.d/20-xdebug.ini
    You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.
  </code></pre>
  
  <p>Can you help me to solve this problem?</p>