Code Modernization: Use correct property in IXR_Message::tag_open().

The `IXR_Message` class declares a property `_currentTag`, which is never assigned or used. It does assign to `currentTag` instead, which outside of that one assignment is never used either.

Since there are various other underscore-prefixed properties declared on the class, including one named `_currentTagContents` which is used in several places, it appears that the declared property is correct and the assignment is a typo.

This commit resolves a notice on PHP 8.2:
{{{
Deprecated: Creation of dynamic property IXR_Message::$currentTag is deprecated
}}}

Follow-up to [1346].

Props bjorsch, kraftbj, jrf, mukesh27, SergeyBiryukov.
See #56790.

git-svn-id: https://develop.svn.wordpress.org/trunk@55105 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/IXR/class-IXR-message.php

@@ -144,7 +144,7 @@ function parse()
     function tag_open($parser, $tag, $attr)
     {
         $this->_currentTagContents = '';
-        $this->currentTag = $tag;
+        $this->_currentTag = $tag;
         switch($tag) {
             case 'methodCall':
             case 'methodResponse':

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 );
+	}
+
+}