Error Message On Password Recovery

In latest YUM v0.9 (php 5.4), after selecting password recovery, entering an email address and hitting Restore, I’m getting the error message:

Fatal error: Call to undefined function md5sum() in ...\public_html\modules\user\models\YumUser.php on line 664

The line is:

      $this->activationKey = md5sum(microtime() . $this->password, Yum::module()->passwordHashCost);

I’ve looked for md5sum(). It’s not a php function, nor is it defined somewhere in the code.

Any ideas?