How to capture error in php if exec() function fails to run some command

How to capture error in PHP if exec() function fails to run some command?

e.g. $command = "ls";
exec($command, $output, $return_value); will give
$return_value = 0 & $output contains list of files.

but if $command = "lk";
In this case error will be "lk: command not found"
But exec() command will not capture error into $output.

So, use this before calling exec() function.
$command .= " 2>&1";

It will redirect the STDERR stream to STDOUT, so that you can capture both OUTPUT as well as ERROR from your PHP script into $output.