What does this php operator do? : <pre><code>$serial = ($_SERVER['SSL_CLIENT_M_SERIAL'] ?? false);
</code></pre>

<p>It looks like a bit like a ternary. But for that I would have expected:</p>

<pre><code>$serial = $_SERVER['SSL_CLIENT_M_SERIAL'] ?  $_SERVER['SSL_CLIENT_M_SERIAL'] : false
</code></pre>

<p>perhaps it's shorthand, but finding a link to confirm is difficult. I found the snippet in php4 legacy code.</p>

<p>(note this is php not c#)</p>
