Comment: Use wp_parse_url() instead of parse_url()

src/wp-includes/comment.php

@@ -2931,7 +2931,7 @@ function discover_pingback_server_uri( $url, $deprecated = '' ) {
 	$pingback_str_squote = 'rel=\'pingback\'';
 
 	/** @todo Should use Filter Extension or custom preg_match instead. */
-	$parsed_url = parse_url( $url );
+	$parsed_url = wp_parse_url( $url );
 
 	if ( ! isset( $parsed_url['host'] ) ) { // Not a URL. This should never happen.
 		return false;

tests/phpunit/tests/comment/discoverPingbackServerUri.php

@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * @group comment
+ * @covers ::discover_pingback_server_uri
+ *
+ * @ticket 31384
+ */
+class Tests_Comment_DiscoverPingbackServerUri extends WP_UnitTestCase {
+
+	public function set_up() {
+		parent::set_up();
+		add_filter( 'pre_http_request', array( $this, 'mock_http_request' ) );
+	}
+
+	public function tear_down() {
+		remove_filter( 'pre_http_request', array( $this, 'mock_http_request' ) );
+		parent::tear_down();
+	}
+
+	public function mock_http_request() {
+		return array(
+			'headers'  => array(),
+			'body'     => '<link rel="pingback" href="https://example.com/pingback" />',
+			'response' => array( 'code' => 200 ),
+			'cookies'  => array(),
+		);
+	}
+
+	/**
+	 * @ticket 31384
+	 */
+	public function test_discovers_pingback_uri_for_schemeless_url() {
+		$result = discover_pingback_server_uri( '//example.com/post' );
+
+		$this->assertSame( 'https://example.com/pingback', $result );
+	}
+}