Code Modernization: Check the return type of parse_url() in WP::parse_request().

As per the PHP manual:
> If the `component` parameter is omitted, an associative array is returned.
> If the `component` parameter is specified, `parse_url()` returns a string (or an int, in the case of `PHP_URL_PORT`) instead of an array. If the requested component doesn't exist within the given URL, `null` will be returned.

Reference: [https://www.php.net/manual/en/function.parse-url.php#refsect1-function.parse-url-returnvalues PHP Manual: parse_url(): Return Values]

In this case, `parse_url()` is called with the `PHP_URL_PATH` as `$component`. This will return `null` in the majority of cases, as – exсept for subdirectory-based sites – `home_url()` returns a URL without the trailing slash, like `http://example.org`.

The return value of `parse_url()` was subsequently passed to `trim()`, leading to a `trim(): Passing null to parameter #1 ($string) of type string is deprecated` notice on PHP 8.1.

Fixed by adjusting the logic flow to:
* Only pass the return value of `parse_url()` to follow-on functions if it makes sense, i.e. if it isn't `null`, nor an empty string.
* Preventing calls to `preg_replace()` and `trim()` further down in the function logic flow, when `preg_replace()`/`trim()` would have nothing to do anyhow.

Follow-up to [25617].

Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51622 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp.php

@@ -170,8 +170,13 @@ public function parse_request( $extra_query_vars = '' ) {
 
 			list( $req_uri ) = explode( '?', $_SERVER['REQUEST_URI'] );
 			$self            = $_SERVER['PHP_SELF'];
-			$home_path       = trim( parse_url( home_url(), PHP_URL_PATH ), '/' );
-			$home_path_regex = sprintf( '|^%s|i', preg_quote( $home_path, '|' ) );
+
+			$home_path       = parse_url( home_url(), PHP_URL_PATH );
+			$home_path_regex = '';
+			if ( is_string( $home_path ) && '' !== $home_path ) {
+				$home_path       = trim( $home_path, '/' );
+				$home_path_regex = sprintf( '|^%s|i', preg_quote( $home_path, '|' ) );
+			}
 
 			/*
 			 * Trim path info from the end and the leading home path from the front.
@@ -180,14 +185,17 @@ public function parse_request( $extra_query_vars = '' ) {
 			 */
 			$req_uri  = str_replace( $pathinfo, '', $req_uri );
 			$req_uri  = trim( $req_uri, '/' );
-			$req_uri  = preg_replace( $home_path_regex, '', $req_uri );
-			$req_uri  = trim( $req_uri, '/' );
-			$pathinfo = trim( $pathinfo, '/' );
-			$pathinfo = preg_replace( $home_path_regex, '', $pathinfo );
 			$pathinfo = trim( $pathinfo, '/' );
 			$self     = trim( $self, '/' );
-			$self     = preg_replace( $home_path_regex, '', $self );
-			$self     = trim( $self, '/' );
+
+			if ( ! empty( $home_path_regex ) ) {
+				$req_uri  = preg_replace( $home_path_regex, '', $req_uri );
+				$req_uri  = trim( $req_uri, '/' );
+				$pathinfo = preg_replace( $home_path_regex, '', $pathinfo );
+				$pathinfo = trim( $pathinfo, '/' );
+				$self     = preg_replace( $home_path_regex, '', $self );
+				$self     = trim( $self, '/' );
+			}
 
 			// The requested permalink is in $pathinfo for path info requests and
 			// $req_uri for other requests.

tests/phpunit/tests/wp/parseRequest.php

@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @group wp
+ * @covers WP::parse_request
+ */
+class Tests_WP_ParseRequest extends WP_UnitTestCase {
+	/**
+	 * @var WP
+	 */
+	protected $wp;
+
+	public function set_up() {
+		parent::set_up();
+		$this->wp = new WP();
+	}
+
+	/**
+	 * Test that PHP 8.1 "passing null to non-nullable" deprecation notice
+	 * is not thrown when the home URL has no path/trailing slash (default setup).
+	 *
+	 * Note: This does not test the actual functioning of the parse_request() method.
+	 * It just and only tests for/against the deprecation notice.
+	 *
+	 * @ticket 53635
+	 */
+	public function test_no_deprecation_notice_when_home_url_has_no_path() {
+		// Make sure rewrite rules are not empty.
+		$this->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
+
+		// Make sure the test will function independently of whatever the test user set in wp-tests-config.php.
+		add_filter(
+			'home_url',
+			static function ( $url ) {
+				return 'http://example.org';
+			}
+		);
+
+		$this->wp->parse_request();
+		$this->assertSame( '', $this->wp->request );
+	}
+}