FFmpeg concat videos from array with fade effect

Question

I am trying to concatenate videos into one using PHP and FFMpeg.

The URL of each video are in the array $files_aux[].

$video = $ffmpeg->open($files_aux[0]); 
$video->filters()->synchronize();
$video
   ->concat($files_aux)
   ->saveFromSameCodecs(__DIR__ . $newfilename, true, true);

It works well, but I want to add a fade effect in between each video. Can someone help?

Thanks in advance