title: ffmpeg strange errors variables/undefined constants
body: <p>I was trying to achieve get an animated text overlay. I found this thread:
  <a href="https://stackoverflow.com/questions/36496855/ffmpeg-moving-text-drawtext/36498026#comment97236974_36498026">ffmpeg moving text drawtext</a></p>
  
  <p>When I run this code however:</p>
  
  <pre><code>ffmpeg -i VideoInput.mp4 \
         -vf "drawtext=enable='between(t,12,15)':fontfile=myfont.otf:text='Test test': \
             x='if(lt(t-t2+1\,0)\,x1+(x2-x1)*(t-t1)/(t2-t1-1)\,x)': \
             y='if(lt(t-t2+1\,0)\,y1+(y2-y1)*(t-t1)/(t2-t1-1)\,y)':fontsize=65" \
         -acodec copy outputVideo.mp4
  </code></pre>
  
  <p>I get the error:</p>
  
  <pre><code>[Parsed_drawtext_0 @ 000002d862595c00][Eval @ 000000e1d79fe300] Undefined constant or missing '(' in 't2-t1-1),x)' 
  [Parsed_drawtext_0 @ 000002d862595c00] Failed to configure input pad on Parsed_drawtext_0
  </code></pre>
  
  <p>A second strange error, which may be the cause for the problem at the top. Is that I can't run the arguments on multiple lines. So with the code below I have to remove the slashes and put it on 1 line. I use PHP so this does not work (white screen, no output)</p>
  
  <pre><code>$arguments = "-i out1.mp4 \
         -vf \"drawtext=enable='between(t,12,15)':fontfile=myfont.otf:text='Test test': \
             x='if(lt(t-t2+1\,0)\,x1+(x2-x1)*(t-t1)/(t2-t1-1)\,x)': \
             y='if(lt(t-t2+1\,0)\,y1+(y2-y1)*(t-t1)/(t2-t1-1)\,y)':fontsize=65\" \
         -acodec copy outputVideo.mp4 2&gt;&amp;1";
  $output = shell_exec('D:/wamp64/www/createslideshow/ffmpeg-20190319-f8075b2-win64-static/bin/ffmpeg.exe '.$arguments);
  </code></pre>
  
  <p>This works but gives the undefined constant error:</p>
  
  <pre><code>$arguments = "-i out1.mp4 -vf \"drawtext=enable='between(t,12,15)':fontfile=myfont.otf:text='Test test': x='if(lt(t-t2+1\,0)\,x1+(x2-x1)*(t-t1)/(t2-t1-1)\,x)': y='if(lt(t-t2+1\,0)\,y1+(y2-y1)*(t-t1)/(t2-t1-1)\,y)':fontsize=65\" -acodec copy outputVideo.mp4 2&gt;&amp;1";
  $output = shell_exec('D:/wamp64/www/createslideshow/ffmpeg-20190319-f8075b2-win64-static/bin/ffmpeg.exe '.$arguments);
  </code></pre>