title: Filter concat:out:v0 has a unconnected output | Concatenating with php-ffmpeg
body: <p>I'm incorporating <a href="https://github.com/PHP-FFMpeg/PHP-FFMpeg" rel="nofollow noreferrer">PHP-FFMpeg</a> to my symfony3 project which is working perfectly. However recently I've been needing to concatenate two videos into one, and it would appear it's not supported. So I created a <code>ConcatVideoFilter</code> class that I'm using based off of <a href="https://gist.github.com/guivazcabral/469ef56b70014c95ca62" rel="nofollow noreferrer">this</a> </p>
  
  <pre><code>    public function apply(Video $video, VideoInterface $format)
      {
          $params = [];
          $count = count($this-&gt;files) + 1;
          foreach ($this-&gt;files as $i =&gt; $file) {
              $params[] = '-i';
              $params[] = $file;
          }
          $params[] = '-filter_complex';
          $params[] = 'concat=n='.$count.':v=1:a=1 [v]';
          $params[] = '-map';
          $params[] = '[v]';
          return $params;
      }
  </code></pre>
  
  <p>I'm getting an encoding failed error in my symfony profiler </p>
  
  <p><a href="https://i.stack.imgur.com/V3qug.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/V3qug.png" alt="RuntimeException: Encoding failed, ExecutionFailureException: ffmpeg failed to execute command"></a></p>
  
  <p>I then copy pasted the actual ffmpeg command line to see what error it would give:</p>
  
  <p><a href="https://i.stack.imgur.com/G6Ej7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/G6Ej7.png" alt="Filter concat:out:v0 has a unconnected output"></a></p>
  
  <p>Which is giving me this error </p>
  
  <blockquote>
    <p>Filter concat:out:v0 has a unconnected output</p>
  </blockquote>
  
  <p>What does this error mean, and what can I do to get rid of it?</p>