title: The requested PHP extension ext-mysql * is missing from your system. in php 7.1.7
body: <p>I am trying to deploy php app on heroku.
  I have added tables to clearDB but stuck on this part:
  after adding </p>
  
  <pre><code>"ext-mysql":"*" 
  </code></pre>
  
  <p>this line  in composer. json it thows me error saying 
  ERROR: Your 'composer.json' lists dependencies inside 'require', but no 'composer.lock' was found. Please run 'composer update' to re-generate 'composer.lock' if necessary, and commit it into your repository. </p>
  
  <p>and after running </p>
  
  <pre><code>php composer.phar update
  </code></pre>
  
  <p>I get this error</p>
  
  <pre><code>The requested PHP extension ext-mysql * is missing from your system. Install or enable PHP's mysql extension.
  </code></pre>