title: Mysqli SELECT only works with numbers, not working with letters
body: <p>I have this query:</p>
  
  <pre><code>SELECT users.email
  FROM prefix_users as users
  LEFT JOIN prefix_user_permission_matches as matches ON users.id = 
  matches.user_id
  LEFT JOIN prefix_permissions as permissions ON permissions.id = matches.permission_id
  WHERE permissions.name IN ($entreprise)
  </code></pre>
  
  <p>The query only works when the search/WHERE parameter $entreprise is numerical, but it does not work with letters.</p>
  
  <p>Any idea why?</p>
  
  <p>I am using the query in PHP with prepared statement. Sorry if the question is poorly written, let me know if you want more info.</p>