Backport: #2315

.github/phpstan-baseline.neon

@@ -5050,6 +5050,16 @@ parameters:
 			count: 1
 			path: ../app/code/core/Mage/Sales/Block/Recurring/Profiles.php
 
+		-
+			message: "#^Variable \\$oldArea might not be defined\\.$#"
+			count: 1
+			path: ../app/code/core/Mage/Sales/Model/Api2/Order/Comment/Rest/Admin/V1.php
+
+		-
+			message: "#^Variable \\$oldStore might not be defined\\.$#"
+			count: 1
+			path: ../app/code/core/Mage/Sales/Model/Api2/Order/Comment/Rest/Admin/V1.php
+
 		-
 			message: "#^Call to an undefined method Varien_Simplexml_Element\\:\\:getClassName\\(\\)\\.$#"
 			count: 1

app/code/core/Mage/Sales/Model/Api2/Order/Comment/Rest.php

@@ -32,6 +32,7 @@ abstract class Mage_Sales_Model_Api2_Order_Comment_Rest extends Mage_Sales_Model
      * Parameters in request used in model (usually specified in route mask)
      */
     public const PARAM_ORDER_ID = 'id';
+    public const PARAM_COMMENT_ID = 'comment_id';
     /**#@-*/
 
     /**

app/code/core/Mage/Sales/Model/Api2/Order/Comment/Rest/Admin/V1.php

@@ -27,4 +27,67 @@
  */
 class Mage_Sales_Model_Api2_Order_Comment_Rest_Admin_V1 extends Mage_Sales_Model_Api2_Order_Comment_Rest
 {
+    /**
+     * Add comment to order
+     *
+     * @param array $data
+     * @return string
+     */
+    protected function _create(array $data)
+    {
+        $orderId = $this->getRequest()->getParam(self::PARAM_ORDER_ID);
+        $order   = $this->_loadOrderById($orderId);
+
+        $status         = $data['status'] ?? false;
+        $comment        = $data['comment'] ?? '';
+        $visibleOnFront = $data['is_visible_on_front'] ?? 0;
+        $notifyCustomer = array_key_exists('is_customer_notified', $data) ? $data['is_customer_notified'] : false;
+
+        $historyItem = $order->addStatusHistoryComment($comment, $status);
+        $historyItem->setIsCustomerNotified($notifyCustomer)
+            ->setIsVisibleOnFront((int) $visibleOnFront)
+            ->save();
+
+        try {
+            if ($notifyCustomer && $comment) {
+                $oldStore = Mage::getDesign()->getStore();
+                $oldArea = Mage::getDesign()->getArea();
+                Mage::getDesign()->setStore($order->getStoreId());
+                Mage::getDesign()->setArea('frontend');
+            }
+
+            $order->save();
+            $order->sendOrderUpdateEmail((bool) $notifyCustomer, $comment);
+
+            if ($notifyCustomer && $comment) {
+                Mage::getDesign()->setStore($oldStore);
+                Mage::getDesign()->setArea($oldArea);
+            }
+        } catch (Mage_Core_Exception $e) {
+            $this->_critical($e->getMessage(), self::RESOURCE_INTERNAL_ERROR);
+        } catch (Throwable $t) {
+            Mage::logException($t);
+            $this->_critical($t->getMessage(), self::RESOURCE_UNKNOWN_ERROR);
+        }
+
+        return $this->_getLocation($historyItem);
+    }
+
+    /**
+     * Retrieve order comment by id
+     *
+     * @return array
+     */
+    protected function _retrieve()
+    {
+        $comment = Mage::getModel('sales/order_status_history')->load(
+            $this->getRequest()->getParam(self::PARAM_COMMENT_ID)
+        );
+
+        if (!$comment->getId()) {
+            $this->_critical(self::RESOURCE_NOT_FOUND);
+        }
+
+        return $comment->getData();
+    }
 }

app/code/core/Mage/Sales/etc/api2.xml

@@ -303,6 +303,7 @@
                 <title>Order Comments</title>
                 <privileges>
                     <admin>
+                        <create>1</create>
                         <retrieve>1</retrieve>
                     </admin>
                     <customer>
@@ -323,6 +324,10 @@
                     </customer>
                 </force_attributes>
                 <routes>
+                    <route_entity>
+                        <route>/orders/comments/:comment_id</route>
+                        <action_type>entity</action_type>
+                    </route_entity>
                     <route_collection>
                         <route>/orders/:id/comments</route>
                         <action_type>collection</action_type>