在PHP中，重音符号（`）（不是单引号）代表什么？

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

Answer 2

It is a shorthand for exec() . The output of the command can be directly used in an expression.
http://php.net/manual/en/language.operators.execution.php

(Please note that you still should apply escapeshellarg() to variables in the command string beforehand.)