Add PHPUnit tests for trackback_response() function

tests/phpunit/tests/trackback/trackbackResponse.php

@@ -0,0 +1,76 @@
+<?php
+
+/**
+ * Test the `trackback_response()` function.
+ *
+ * @group trackback
+ * @covers ::trackback_response
+ */
+class Tests_Trackback_trackbackResponse extends WP_UnitTestCase {
+
+	/**
+	 * Post used to load wp-trackback.php and define trackback_response().
+	 *
+	 * @var int
+	 */
+	private $post_id;
+
+	/**
+	 * Loads wp-trackback.php with a valid request so its function definitions are available.
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		global $wp, $wpdb;
+
+		$this->post_id = $this->factory()->post->create();
+		$wp            = new stdClass();
+		$post_id       = $this->post_id;
+		$old_get       = $_GET;
+		$old_post      = $_POST;
+
+		$_GET  = array( 'tb_id' => $this->post_id );
+		$_POST = array(
+			'url'       => 'https://example.com/trackback',
+			'title'     => 'Trackback title',
+			'blog_name' => 'Trackback blog',
+		);
+
+		if ( ! function_exists( 'trackback_response' ) ) {
+			set_error_handler(
+				static function ( $severity, $message ) {
+					return E_WARNING === $severity && str_contains( $message, 'Cannot modify header information' );
+				}
+			);
+			ob_start();
+			require ABSPATH . 'wp-trackback.php';
+			ob_end_clean();
+			restore_error_handler();
+		}
+
+		$_GET  = $old_get;
+		$_POST = $old_post;
+	}
+
+	/**
+	 * Tests that trackback_response() returns a successful XML response.
+	 *
+	 * @ticket 66011
+	 */
+	public function test_trackback_response_success() {
+		set_error_handler(
+			static function ( $severity, $message ) {
+				return E_WARNING === $severity && str_contains( $message, 'Cannot modify header information' );
+			}
+		);
+		ob_start();
+		trackback_response();
+		$response = ob_get_clean();
+		restore_error_handler();
+
+		$this->assertSame(
+			"<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<response>\n<error>0</error>\n</response>",
+			$response
+		);
+	}
+}