Truncated MailFrom address - Incorrect Windows detection

The "from" email address of messages sent by Drupal (contact form, password reset, etc..) is trimmed of 5 characters on the left when running Drupal on Windows Server 2008 R2 with IIS 7.5 and PHP 7.1 64-bit (FastCGI).

"contact@domain.ext" is changed to "ct@domain.ext", and "info@domain.ext" becomes "domain.ext"

The problem has happened after upgrading PHP from 5.x to 7.1. PHP is run via FastCGI, and PHP 7.1 is installed in 64-bit version. 5.x versions were only available in 32-bit.

PHP mail() function works a bit differently on Windows and Drupal mail function (in system.mail.inc) includes a workaround :

if (!isset($_SERVER['WINDIR']) && strpos($_SERVER['SERVER_SOFTWARE'], 'Win32') === FALSE) {
  // default processing
}
else {
  // Windows processing - set and reset sendmail_from
}

I think that this workaround is rarely used, since $_SERVER['SERVER_SOFTWARE'] value is "Microsoft-IIS/x.y" x and y being major and minor version of IIS, on Windows servers running IIS, and not Win32

If I change Windows detection code to use PHP_OS, messages sent by Drupal are OK, with complete "from" address.

if (!isset($_SERVER['WINDIR']) && strpos(strtolower(substr(PHP_OS, 0, 3)), 'win') === FALSE) {
  // default processing
}
else {
  // Windows processing - set and reset sendmail_from
}