How to concatenate two .ogg (opus) files with ffmpeg exec php?

Question

PHP

I have 2 files a1.ogg and a2.ogg (OPUS codec)
I need to merge these files to get a new audio file
I try to execute

exec ("ffmpeg -i 'concat:a1.ogg|a2.ogg' -c copy out.ogg",$output);
var_dump($output);

file not created
result

array(0) { }

I checked the work on mp3 files, gluing also did not occur

exec('ffmpeg -i "concat:1.mp3|2.mp3" -c copy 3.mp3',$output);

The only thing that worked
$ ffmpeg -i input.mp4 output.avi

UPD
As I found out, there is no opus codec in the ffmpeg assembly. And the source files are in this format.