title: What does the grave accent symbol (`) ( not single quote) represent in PHP?
body: <p>In the example below, what do the grave accents in the second line mean?</p>
  
  <pre><code>$cmd = "$ffmpeg -i $video -deinterlace -an -ss $second -t 00:00:01 -r 1 -y -vcodec mjpeg -f mjpeg $image 2&gt;&amp;1";
  
  $return = `$cmd`
  </code></pre>