Questions about exec function return values

Exec function return value: string & nbsp; exec & nbsp; (& nbsp; string & nbsp; $ command & nbsp; [, & nbsp; array & nbsp; & amp; $ output & nbsp; [, & nbsp; int & nbsp; & amp; $ return_v
Exec function prototype:
String exec (string $ command [, array & $ output [, int & $ return_var])
Why should I add "&" at the end of $ command, and $ return_var is always 0?

------ Solution --------------------
$ Return_var if it is 0, the command is successfully executed.
------ Solution --------------------
The & indicates execution in the background. the returned result is whether the result is successfully placed in the background.