Code Modernization: Correct handling of null in wp_parse_str().

This fixes `parse_str(): Passing null to parameter #1 ($string) of type string is deprecated` notices on PHP 8.1, without change in behaviour.

Impact: 311 of the pre-existing tests are affected by this issue.

The PHP native `parse_str()` function expects a string, however, based on the failing tests, it is clear there are functions in WordPress which passes a non-string – including `null` – value to the `wp_parse_str()` function, which would subsequently pass it onto the PHP native function without further input validation.

Most notable offender is the `wp_parse_args()` function which special cases arrays and objects, but passes everything else off to `wp_parse_str()`.

Several ways to fix this issue have been explored, including checking the received value with `is_string()` or `is_scalar()` before passing it off to the PHP native `parse_str()` function.

In the end it was decided against these in favor of a string cast as:
* `is_string()` would significantly change the behavior for anything non-string.
* `is_scalar()` up to a point as well, as it does not take objects with a `__toString()` method into account.

Executing a string cast on the received value before passing it on maintains the pre-existing behavior while still preventing the deprecation notice coming from PHP 8.1.

Reference: [https://www.php.net/manual/en/function.parse-str.php PHP Manual: parse_str()]

Follow-up to [5709].

Props jrf, hellofromTonya, lucatume, SergeyBiryukov.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51624 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/formatting.php

@@ -4964,7 +4964,7 @@ function map_deep( $value, $callback ) {
  * @param array  $array  Variables will be stored in this array.
  */
 function wp_parse_str( $string, &$array ) {
-	parse_str( $string, $array );
+	parse_str( (string) $string, $array );
 
 	/**
 	 * Filters the array of variables derived from a parsed string.

tests/phpunit/tests/formatting/wpParseStr.php

@@ -0,0 +1,144 @@
+<?php
+
+/**
+ * @group formatting
+ * @covers ::wp_parse_str
+ */
+class Tests_Formatting_wpParseStr extends WP_UnitTestCase {
+
+	/**
+	 * Tests parsing of a string into variables.
+	 *
+	 * Note: While the function under test does not contain any significant logic,
+	 * these tests document the behavior and safeguard PHP cross-version compatibility.
+	 *
+	 * @dataProvider data_wp_parse_str
+	 *
+	 * @param mixed $input    Value to parse.
+	 * @param array $expected Expected function output.
+	 */
+	public function test_wp_parse_str( $input, $expected ) {
+		wp_parse_str( $input, $output );
+		$this->assertSame( $expected, $output );
+	}
+
+	/**
+	 * Data Provider.
+	 *
+	 * @return array
+	 */
+	public function data_wp_parse_str() {
+		return array(
+			'null'              => array(
+				'input'    => null,
+				'expected' => array(),
+			),
+			'boolean false'     => array(
+				'input'    => false,
+				'expected' => array(),
+			),
+			'boolean true'      => array(
+				'input'    => true,
+				'expected' => array(
+					1 => '',
+				),
+			),
+			'integer 0'         => array(
+				'input'    => 0,
+				'expected' => array(
+					0 => '',
+				),
+			),
+			'integer 456'       => array(
+				'input'    => 456,
+				'expected' => array(
+					456 => '',
+				),
+			),
+			'float 12.53'       => array(
+				'input'    => 12.53,
+				'expected' => array(
+					'12_53' => '',
+				),
+			),
+			'plain string'      => array(
+				'input'    => 'foobar',
+				'expected' => array(
+					'foobar' => '',
+				),
+			),
+			'query string'      => array(
+				'input'    => 'x=5&_baba=dudu&',
+				'expected' => array(
+					'x'     => '5',
+					'_baba' => 'dudu',
+				),
+			),
+			'stringable object' => array(
+				'input'    => new Fixture_Formatting_wpParseStr(),
+				'expected' => array(
+					'foobar' => '',
+				),
+			),
+		);
+	}
+
+	/**
+	 * Tests that the result array only contains the result of the string parsing
+	 * when provided with different types of input for the `$output` parameter.
+	 *
+	 * @dataProvider data_wp_parse_str_result_array_is_always_overwritten
+	 *
+	 * @param array|null $output   Value for the `$output` parameter.
+	 * @param array      $expected Expected function output.
+	 */
+	public function test_wp_parse_str_result_array_is_always_overwritten( $output, $expected ) {
+		wp_parse_str( 'key=25&thing=text', $output );
+		$this->assertSame( $expected, $output );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_wp_parse_str_result_array_is_always_overwritten() {
+		// Standard value for expected output.
+		$expected = array(
+			'key'   => '25',
+			'thing' => 'text',
+		);
+
+		return array(
+			'output null'                                 => array(
+				'output'   => null,
+				'expected' => $expected,
+			),
+			'output empty array'                          => array(
+				'output'   => array(),
+				'expected' => $expected,
+			),
+			'output non empty array, no conflicting keys' => array(
+				'output'   => array(
+					'foo' => 'bar',
+				),
+				'expected' => $expected,
+			),
+			'output non empty array, conflicting keys'    => array(
+				'output'   => array(
+					'key' => 'value',
+				),
+				'expected' => $expected,
+			),
+		);
+	}
+}
+
+/**
+ * Fixture for use in the tests.
+ */
+class Fixture_Formatting_wpParseStr {
+	public function __toString() {
+		return 'foobar';
+	}
+}