What does the grave accent symbol (`) ( not single quote) represent in PHP?

Question

In the example below, what do the grave accents in the second line mean?

$cmd = "$ffmpeg -i $video -deinterlace -an -ss $second -t 00:00:01 -r 1 -y -vcodec mjpeg -f mjpeg $image 2>&1";

$return = `$cmd`

Answer 1

It executes a shell command. So it executes whatever is in $cmd.

See backtick operator