Problem solved with the following command:
$WshShell = new COM("WScript.Shell");
$oExec = $WshShell->Run("C:\wamp\bin\php\phpVERSIONNUMBER\php-win.exe -f C:/wamp/www/path/to/backgroundProcess.php", 0, false);
Problem solved with the following command:
$WshShell = new COM("WScript.Shell");
$oExec = $WshShell->Run("C:\wamp\bin\php\phpVERSIONNUMBER\php-win.exe -f C:/wamp/www/path/to/backgroundProcess.php", 0, false);