将命令放在后台并通过PHP exec（）获取PID？

When I manually run this command via SSH:

xx@xxx.com [~/public_html/xxx]# ls > ls2.out 2>&1 &
[1] 15205

Which simply returns the PID of the background process which in this case is 15205. However when I tried to do the same via PHP code:

$run = "ls > ls2.out 2>&1 &";
$return = exec($run, $output, $return_var);
echo '=====', PHP_EOL;
echo var_dump($run);
echo var_dump($return);
echo print_r($output);
echo print_r($return_var);
echo '=====', PHP_EOL;

It doesn't return the PID but just an empty string:

=====
string(19) "ls > ls2.out 2>&1 &"
string(0) ""
Array
(
)
101=====

Why?

How can I get the PID of the background process via PHP exec()?

use the proc_open for subprocess, and proc-get-status to get the pid. for exec refer to this post.

<?php
    $descriptorspec = array(
        0 => array("pipe", "r"),
        1 => array("pipe", "w"),
        2 => array("pipe", "w")
    );
    $command = 'ping -c 10 stackoverflow.com';

    $process = proc_open($command, $descriptorspec, $pipes);
    echo 'hahaha';