Execute command by php exec() with xampp in ubuntu {not working}

I executed this command nohup ffmpeg -i input.mp4 -vcodec h264 -preset:v ultrafast -b:v 1000k -acodec mp3 output.mp4 2>output.txt & in ubuntu by Terminal and it worked properly but when I tried to execute it by php exec($myCommand) it didn't work and show this message

Error message: enter image description here

I tried to solve it by change name of this file libstdc++.so.6 to libstdc++.so.6.old in this directory /opt/lampp/lib after that when I execute my command it show this message ffmpeg: symbol lookup error: /lib/x86_64-linux-gnu/libcairo.so.2: undefined symbol: FT_Get_Var_Design_Coordinates