Update AssignHandler.php

Author: staabm

src/Analyser/ExprHandler/AssignHandler.php

@@ -1013,6 +1013,8 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 				}
 
 			} else {
+				// when $unionValues=false the array-item type will be replaced with $valueToWrite
+				// when $unionValues=true the existing array item-type will be union with $valueToWrite
 				$unionValues = false;
 				if ($i === 0) {
 					$unionValues = true;