title: Using HTML5 validation feature to check for existing username
body: <p>I'm creating a registration form and I would like to check if the user already exist using PHP and HTML5 validation. </p>
  
  <p>So somehow I would like to send the username that was typed in to a PHP function and write error message with <code>setCustomValidity()</code> in JavaScript (HTML5)</p>
  
  <p>Can anyone offer me any advice to achieve this?</p>