Changed the reserved keyword (#56)

* Changed the reserved keyword and added comments to explain potential future changes.

* Fixed the bucketing ID reserved name.

Author: alda-optimizely

src/Optimizely/DecisionService/DecisionService.php

@@ -29,7 +29,11 @@
 use Optimizely\UserProfile\UserProfileUtils;
 use Optimizely\Utils\Validator;
 
-define("RESERVED_ATTRIBUTE_KEY_BUCKETING_ID",     "Optimizely Bucketing ID");
+// This value was decided between App Backend, Audience, and Oasis teams, but may possibly change.
+// We decided to prefix the reserved keyword with '$' because it is a symbol that is not
+// allowed in custom attributes.
+// We also thought that the prefix 'opt' makes it apparent to users that the variable belongs to Optimizely.
+define("RESERVED_ATTRIBUTE_KEY_BUCKETING_ID",     "\$opt_bucketing_id");
 
 /**
  * Optimizely's decision service that determines which variation of an experiment the user will be allocated to.

src/Optimizely/Event/Builder/EventBuilder.php

@@ -106,6 +106,8 @@ private function setCommonParams($config, $userId, $attributes)
             if ($attributeValue) {
                 // check for reserved attributes
                 if (strcmp($attributeKey , RESERVED_ATTRIBUTE_KEY_BUCKETING_ID) == 0) {
+                    // TODO (Alda): the type for bucketing ID attribute may change so that custom
+                    // attributes are not overloaded
                     array_push($this->_eventParams[USER_FEATURES], [
                         'name' => RESERVED_ATTRIBUTE_KEY_BUCKETING_ID_EVENT_PARAM_KEY,
                         'type' => 'custom',