title: Client Server REST API captcha implementation
body: <p>I'm building client server REST application. 
  Client side is based on Angular while server is PHP (not that it matters much anyhow).</p>
  
  <p>What I am wondering if there are any best practices, good examples of captcha implementation in this case? Captcha would be used for user registration etc.</p>
  
  <p>I'm not limited to any specific libraries, only requirement is that there cannot be any calls to 3rd party servers on client side (js libraries hosted on 3rd party servers or req api key etc).</p>
  
  <p>thanks</p>