Accessing Files on Windows Domain Share Drive in PHP

I have a bunch of documents stored on a separate machine that I need to serve from a PHP/IIS server. The catch is that the document drive is on a machine that's attached to an Active Directory domain, and the webserver is not, so I'm unsure where to provide the AD user that PHP needs to authenticate with to access the drive.

I have the IIS application pool that calls PHP running as a new Windows user I created (set as the app pool's "Identity", and as the web site's Anonymous Authentication user). I've logged into the server as this new user and set up a Mapped Network Drive, but after some research I think that's a dead-end; I read that the service doesn't log into Windows in a way that loads mapped drives.

Referencing just by IP address is closer to working, except I get authenticating errors. The main solution I'm finding online is to make the app pool run as a user who has access to the drive, but I don't know if that's possible since the server isn't on that Windows domain. (But I'm also not an AD expert)

echo shell_exec('whoami'); //Outputs the name of the Windows user I created

echo scandir('\\\\192.168.1.120\\data$'); //Username/password not found
echo trim(shell_exec('dir \\\\192.168.1.120\\data$ 2>&1')); //Username or password is incorrect

echo scandir('Z:\\'); //Path not found PHP warning
echo trim(shell_exec('dir Z: 2>&1')); //Path not found

If there isn't an easy, non-hacky solution, I can see if the client can put the webserver on the AD domain, but I don't know if that'll have unforeseen side-effects on the other services this server runs.