From :

You'll need to use an array to get output.

So, use the following code:

$result=[]; // Create an array
exec('your command line', &$result);  // Remember the & before the result as it ***IS A REFERENCE***
//DO whatever with your result

The problem is that you didn't pass your result array as a reference.

Or, use shell_exec, where your return value is everything returned:

$return=shell_exec($command_line);

You can also use backticks:

$result=`cmdline`

Or, use popen(:

Create a new pointer with:

$handle = popen("/bin/ls", "r");

and then read it:

$read = fread($handle);

and close with:

pclose($handle);

It will block execution until the output stops.

PHP functions like exec() or shell_exec() won't help because libvirt executes in root mode, while PHP executes as www-data user. We can give www-data root privileges, but there may be some security issues in that case. So best way is to use libvirt-php API. For example, to list all the VM in php:

<?php
     $conn = libvirt_connect('null', false);
     $doms = libvirt_list_domains($conn);
     print_r($doms);
?>

References:

This will execute smoothly if libvirt is listening on a TCP port. To make libvirt listening on a tcp port, this question will help: I can't use libvirt with listen TCP