title: the requested PHP extension mbstring is missing from your system
body: <p>my error message that I am getting from the composer is:
  Your requirements could not be resolved to an installable set of packages.</p>
  
  <pre><code> Problem 1
      - Installation request for laravel/framework v5.4.28 -&gt; satisfiable by laravel/framework[v5.4.28].
      - laravel/framework v5.4.28 requires ext-mbstring * -&gt; the requested PHP extension mbstring is missing from your system.
    Problem 2
      - Installation request for phpunit/phpunit 5.7.21 -&gt; satisfiable by phpunit/phpunit[5.7.21].
      - phpunit/phpunit 5.7.21 requires ext-mbstring * -&gt; the requested PHP extension mbstring is missing from your system.
    Problem 3
      - laravel/framework v5.4.28 requires ext-mbstring * -&gt; the requested PHP extension mbstring is missing from your system.
      - laravel/tinker v1.0.1 requires illuminate/console ~5.1 -&gt; satisfiable by laravel/framework[v5.4.28].
      - Installation request for laravel/tinker v1.0.1 -&gt; satisfiable by laravel/tinker[v1.0.1].
  </code></pre>
  
  <p>I have tried uncommenting the extension=php_mbstring.dll but it still raises the error. I have tried to change the extension_dir in the "php.ini" and it gives an error. Help is appreciated. </p>