I'm attaching a patch to fix and cleanup various items found by HHVM. Most of these changes are trivial, except for the last one – fix getRates().

• includes/discovery/functions.inc.php
  • Fix reference to sensor_id and add global $debug in check_valid_sensors().
• includes/functions.php
  • Make $snmpver an optional parameter on add_device() since it can be empty when discovering devices.
• poll-billing.php
  • Add global $options to poll_bill().
• includes/rewrites.php
  • Fix reference to the number of entities in humanize_alert_check().
  • Remove invalid extra parameter to format_timestamp().
• includes/billing.php
  • Add global $device to getValues().
  • Quote array keys instead of using constant-style bare words in getLastPortCounter() and getLastMeasurement().
  • Fix variable naming bug in getRates(). How did this go unnoticed?