title: What does the HTTP header If-None-Match: * mean?
body: <p>What does the following HTTP 1.1 header mean?</p>
  
  <blockquote>
    <p>If-None-Match: *</p>
  </blockquote>
  
  <p>I understand it when using a strong or weak ETag or even a list of ETags, but I don't understant it's use when using star (*).</p>
  
  <p><strong>Edit:</strong> Would be nice to have some pseudocode (PHP code good also) that would show how/what to answer to "If-None-Match: *".</p>