Why array can be used as callback
<p>On this page of PHP manual <a href="http://www.php.net/manual/en/function.session-set-save-handler.php" rel="nofollow">http://www.php.net/manual/en/function.session-set-save-handler.php</a>, I found this function spec: <code>bool session_set_save_handler ( callable $open , callable $close , callable $read , callable $write , callable $destroy , callable $gc )</code> which indicates all parameters should be a callback. But I also found this example on that page:</p>

<pre><code>    class FileSessionHandler
    {
        private $savePath;

        function open($savePath, $sessionName)
        {
            $this-&gt;savePath = $savePath;
            if (!is_dir($this-&gt;savePath)) {
                mkdir($this-&gt;savePath, 0777);
            }

            return true;
        }

        function close()
        {
           ...
        }
        ...
    }
$handler = new FileSessionHandler();
session_set_save_handler(
    array($handler, 'open'),
    array($handler, 'close'),
    array($handler, 'read'),
    array($handler, 'write'),
    array($handler, 'destroy'),
    array($handler, 'gc')
    );
</code></pre>

<p>Every parameter in the <code>session_set_save_handler</code> is an array in which first element is the handler object and second a string. </p>

<p>Why those arrays can be used as callbacks? I referenced Array, callback manuals but found no clue on how this is possible.</p>
