As suggested before, you'll likely have to use a full assembly string including the assembly name, the assembly version (not the dll file version), the culture, and the public key token. Otherwise, you'll get a 'file not found' catchable error. To quickly find this on your own system to get the ball rolling, open C:\Windows\assembly in explorer (not in cmd as that shows a different view), scroll down the list until you find the assembly you want to work with, hover the mouse over it, and the tooltip shows the exact string to use (though processcorArchitecture if shown is optional). You'll have to use .net utilities or other methods to automate collecting this info on other machines.
You'll also have to use the fully-qualified class name. In other words, if you want to use the 'Form' class within 'System.Windows.Forms', you have to use the full class hierarchy 'System.Windows.Forms.Form'. Otherwise you'll get an unhelpful 'Failed to instantiate .Net object' catchable error.
The following example shows the correct syntax to use the 'Form' class to pop up a blank form from php-cli, which can be used in leu of many Windows GUI PHP extensions out there. Replace Version with your local version, and PublicKeyToken with your own local key token.
<?php
$full_assembly_string = 'System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=a8425bc35256e463';
$full_class_name = 'System.Windows.Forms.Form';
$form = new DOTNET($full_assembly_string, $full_class_name);
$form->Show();
$form_event = '';
while($form_event !== 'close') {
}
?>