title: simplesamlphp IDP Initiated authentication using existing session
body: <p>I have a Zend project where users login through a login page backed by a database. I have implemented IdP initiated SSO functionality using simplesamlphp. </p>
  
  <p>Current flow of the events is :-</p>
  
  <ol>
  <li>User login into my website.</li>
  <li>Clicks on the IDP initiated SSO link.</li>
  <li>User is redirected to the login page I created for SSO login handling. ( Ideally it should happen if user is not logged in. )</li>
  <li>After login user is sent to the required IDP.</li>
  <li>After printing $_SESSION here and there I found that this is happening because proper session variables for simplesamlphp is not set.</li>
  </ol>
  
  <p>What I want simplesaml to do is following :- </p>
  
  <ol>
  <li>User login into to my website.</li>
  <li>User click on IdP initiated SSO link.</li>
  <li>User is redirected to the service provider.</li>
  </ol>
  
  <p>But I am unable to find any method to do so. Please help.</p>