I use the command 
 ffmpeg-I / home/video/a.mp4-r 1-s 950x520-f image2 / home/video/a.jpg 
 can be executed on the terminal, 
 but in the PHP file, using shell_exec ("ffmpeg-I / home/video/a.mp4-r 1-s 950x520-f image2 / home/video/a.jpg") is not successful. 
 Baidu on the Internet said it was a permission problem, so it was tested 
 echo shell_exec ("ls") can execute 
 echo shell_exec (" whoami") to display nginx 
so vim / etc/sudoers adds
< H2 > Allow root to run any commands anywhere < / H2 > root ALL= (ALL) ALL 
 nginx ALL= (ALL) ALL 
after saving, reboot restarts but it doesn"t work.
should I add nginx to the root user group?
