"The handle is invalid" when invoking PSExec using PHP's exec function

Question

Alright, I've read all the forums and other SO posts but to no avail. Been banging my head on the wall for this. I have a very minimal PHP page like this:

 <?php
    exec('PsExec.exe -accepteula \\\\192.168.10.11 -u "administrator" -p "Ph0nYP@sSw0rd" hostname', $output, $returnVar);   
    print_r($output);
    print_r('Return value is: ' . $returnVar);
?>

It is supposed to simply execute the hostnamecommand on 192.168.10.11. I execute the page using php.exe mypage.php but this always results in the following:

Couldn't access 192.168.10.11:
The handle is invalid.
Connecting to 192.168.10.11...
PsExec v2.2 - Execute processes remotely
Copyright (C) 2001-2016 Mark Russinovich
Sysinternals - www.sysinternals.com

• I have verified that PSExec works by running the same command from outside of the php page i.e. PsExec.exe \\192.168.10.11 -u "administrator" -p "Ph0nYP@sSw0rd" hostname

• I have verified that the administrator account has access to the
  target machine 192.168.10.12 in Control Panel > User Accounts > Give other users access to this computer

• I have watched a wireshark capture when I execute the PHP page but there is no traffic. So I don't know if it's even successfully launching PSExec.

• I have verified the credentials work by going to Run >
  \\192.168.10.12\c$ and entering the creds. I am able to browse the
  folder.

• I have tried using PHP's shell_exec and get the same results

So I know the account has access, the creds are good, and that PSExec is working. It seems that once I wrap it inside a php page that something breaks. What does "The handle is invalid" mean here? And what else can I do to troubleshoot or work around it? My goal is to ultimately execute a powershell script on the target box that will use parameters in a POST request to the page. While I'm not married to my current method I'm still very curious as to why it's failing.