Sql-server – db connection uses an anonymous logon instead of the domain user

I have this PHP code running under IIS:

$serverName = "someServer\someServer";
$connectionInfo = array( "Database"=>"SOME_DB");

$conn = sqlsrv_connect( $serverName, $connectionInfo);

if( $conn ) {
     echo "Connection established.<br />";
}else{
     echo "Connection could not be established.<br />";
     die( print_r( sqlsrv_errors(), true));
}

When I try to visit the page, I get the error:

[Microsoft][SQL Server Native Client 10.0][SQL Server]Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'.

Which is confusing, as the documentation here says "The connection will be attempted using Windows Authentication".

The flow of this is:

1. User using x Windows account on workstation1 visits this website hosted on server1
2. server1 executes above PHP code and tries to retrieve data from server2 using y Windows account (y is defined in the IIS settings, under Application Pools->Identity)
3. Windows account y has access to the database

The IIS pool is running under a Windows domain user "domainadm" which has access to the SOME_DB database and the server it sits on.

Why is it trying to authenticate anonymously and how can I fix this so it runs under the user the IIS pool is running as?

In php.ini:

fastcgi.impersonate = 1;

Checking the Authentication section in IIS, the only options I have are "Anonymous Authentication" or "ASP.NET Impersonation". No Windows authentication?

Anonymous is currently enabled, which gives the error above. If I switch to the ASP.NET one I instead get this error when visiting the page:

500 – Internal server error. There is a problem with the resource you are looking for, and it cannot be displayed.