title: HTTP2 headers compatibility with older browsers
body: <p>I heard today that soon the http2 protocol will be implemented in modern browsers. More information: <a href="https://en.wikipedia.org/wiki/HTTP/2" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/HTTP/2</a>, I know wikipedia is not the best resource, but it will give a little clue about what is going on. The question is:</p>
  <hr />
  <p><strong>How will old browsers respond to http2 headers?</strong></p>
  <p>I mean on php (<a href="http://php.net" rel="nofollow noreferrer">http://php.net</a>) there is still (26.02.2015) a link in header function (<a href="http://php.net/manual/en/function.header.php" rel="nofollow noreferrer">http://php.net/manual/en/function.header.php</a>) to the <code>http1.1</code> specification (<a href="http://www.faqs.org/rfcs/rfc2616" rel="nofollow noreferrer">http://www.faqs.org/rfcs/rfc2616</a>). I understand that in <code>http2</code> all I must do is change header from for example <code>HTTP/1.1 404 Not Found</code> to something similar to <code>HTTP/2.0 404 Not Found</code>. But how will older browsers respond to it? Is this transparent for webdevelopers, and php coders, and implemented on browser / server side, or there are some important things / clues about compatibility?</p>
  <p><strong>Is using http2 headers right after they are ready a good idea?</strong></p>
  <p>I don't want to hurt anyone, but I know such a browser that it first name starts on the leter <code>I</code>, and second with the letter <code>E</code>, that can always mess up a little. I'm afraid that the new specification will totally ruined all older version of this browser, and that this <code>http2</code>. And we - developers must write websites that just work, no matter where, and the magic <code>http2</code> will come true after milions tons of patches / upgrades / months of compatibility problems with older machines.</p>
  <hr />
  <p>There must be some code in well formed question, so here it is :):</p>
  <pre><code>&lt;?php
      header(&quot;HTTP/2.0 404 Not Found&quot;); // Am I correct? It will look like this?
  ?&gt;
  </code></pre>
  <p>What about old browsers in this case?</p>
  <p>Is this a good idea to use it right after http2 is alive?</p>
  <hr />
  <p>Additional documentation:</p>
  <ol>
  <li>Draft http2 specification: <a href="https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-http2-17" rel="nofollow noreferrer">https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-http2-17</a></li>
  <li>Wikipedia few words: <a href="https://en.wikipedia.org/wiki/HTTP/2" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/HTTP/2</a></li>
  <li>PHP header function: <a href="http://php.net/manual/en/function.header.php" rel="nofollow noreferrer">http://php.net/manual/en/function.header.php</a></li>
  <li>W3.org about http2: <a href="http://www.w3.org/Protocols/HTTP/HTTP2.html" rel="nofollow noreferrer">http://www.w3.org/Protocols/HTTP/HTTP2.html</a></li>
  </ol>