diff --git a/src/wp-includes/IXR/class-IXR-message.php b/src/wp-includes/IXR/class-IXR-message.php
index 6c1279e69c..2b27293f52 100644
--- a/src/wp-includes/IXR/class-IXR-message.php
+++ b/src/wp-includes/IXR/class-IXR-message.php
@@ -144,7 +144,7 @@ class IXR_Message
     function tag_open($parser, $tag, $attr)
     {
         $this->_currentTagContents = '';
-        $this->currentTag = $tag;
+        $this->_currentTag = $tag;
         switch($tag) {
             case 'methodCall':
             case 'methodResponse':
diff --git a/tests/phpunit/tests/xmlrpc/message.php b/tests/phpunit/tests/xmlrpc/message.php
new file mode 100644
index 0000000000..ac68552902
--- /dev/null
+++ b/tests/phpunit/tests/xmlrpc/message.php
@@ -0,0 +1,34 @@
+<?php
+/**
+ * Unit tests covering IXR_Message functionality.
+ *
+ * @package    WordPress
+ * @subpackage IXR
+ */
+
+/**
+ * Test wp-includes/IXR/class-IXR-message.php
+ *
+ * @group xmlrpc
+ */
+class Tests_XMLRPC_Message extends WP_UnitTestCase {
+
+	/**
+	 * Tests that `IXR_Message::tag_open()` does not create a dynamic `currentTag` property,
+	 * and uses the declared `_currentTag` property instead.
+	 *
+	 * The notice that we should not see:
+	 * `Deprecated: Creation of dynamic property IXR_Message::$currentTag is deprecated`.
+	 *
+	 * @ticket 56033
+	 *
+	 * @covers IXR_Message::tag_open
+	 */
+	public function test_tag_open_does_not_create_dynamic_property() {
+		$message = new IXR_Message( '<methodResponse><params><param><value>1</value></param></params></methodResponse>' );
+		$this->assertTrue( $message->parse() );
+		$this->assertSame( 'methodResponse', $message->messageType ); // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+		$this->assertSame( array( '1' ), $message->params );
+	}
+
+}