title: Failed to read session data on PHP7.1
body: <p>Sharing a problem that I had (and now solved).</p>
  
  <p>On my development machine, I run IIS with PHP. I upgraded to PHP7 and suddenly my code no longer worked, returning this error...</p>
  
  <blockquote>
    <p>session_start(): Failed to read session data: user (path: C:\WINDOWS\temp)</p>
  </blockquote>
  
  <p>It looks like a permissions issue, right?  So, I spent a long time tweaking php.ini settings and trying to change folder permissions - with no success.</p>
  
  <p>Then I realised something.  See my answer below.</p>