get-bug.php

<?php

$field_list = PhabricatorCustomField::getObjectFields(
  $revision,
  PhabricatorCustomField::ROLE_DEFAULT
);

$fields = $field_list->getFields();
foreach($fields as $field_key => $value) {
  if($field_key === 'differential:bugzilla-bug-id') {
    echo 'FOUND !!!!'.$field_key.' ('.$value->getValue().')';
    return $value->getValue(); // Outputs "null" despite the value being "1" on the revision detail page
  }
  else {
    echo 'FAILED KEY CHECK: '.$field_key;
  }
}

// No bug number found...how?!
return 0;