MDL-79890 core: Add polyfill for array_keys with filter

This is a Moodle version of a proposed function `array_keys_filter`.

Since this method has not yet been created in upstream PHP, I have
elected to name it with a Moodle prefix as the signature is not known.

In the future, if and when this method is created, we can replace its
content with new method and deprecate it.

lib/moodlelib.php

@@ -11037,3 +11037,29 @@ function site_is_public() {
 function exceeds_password_length(string $password, int $pepperlength = 0): bool {
     return (strlen($password) > (MAX_PASSWORD_CHARACTERS + $pepperlength));
 }
+
+/**
+ * A helper to replace PHP 8.3 usage of array_keys with two args.
+ *
+ * There is an indication that this will become a new method in PHP 8.4, but that has not happened yet.
+ * Therefore this non-polyfill has been created with a different naming convention.
+ * In the future it can be deprecated if a core PHP method is created.
+ *
+ * https://wiki.php.net/rfc/deprecate_functions_with_overloaded_signatures#array_keys
+ *
+ * @param array $array
+ * @param mixed $filter The value to filter on
+ * @return array
+ */
+function moodle_array_keys_filter(array $array, mixed $filter, bool $strict): array {
+    return array_keys(array_filter(
+        $array,
+        function($value, $key) use ($filter, $strict): bool {
+            if ($strict) {
+                return $value === $filter;
+            }
+            return $value == $filter;
+        },
+        ARRAY_FILTER_USE_BOTH,
+    ));
+}

lib/tests/moodlelib_test.php

@@ -5445,4 +5445,87 @@ EOT;
         $this->assertEquals($expected, is_proxybypass($url));
     }
 
+    /**
+     * Test that the moodle_array_keys_filter method behaves in the same way
+     * that array_keys behaved before Moodle 8.3.
+     *
+     * @dataProvider moodle_array_keys_filter_provider
+     * @param array $array
+     * @param mixed $filter
+     * @param bool $strict
+     * @param array $expected
+     * @covers ::moodle_array_keys_filter
+     */
+    public function test_moodle_array_keys_filter(
+        array $array,
+        mixed $filter,
+        bool $strict,
+        array $expected,
+    ): void {
+        $this->assertSame(
+            $expected,
+            moodle_array_keys_filter($array, $filter, $strict),
+        );
+    }
+
+    /**
+     * Data provider for moodle_array_keys_filter tests.
+     *
+     * @return array
+     */
+    public static function moodle_array_keys_filter_provider(): array {
+        return [
+            [['a', 'b', 'c'], 'b', false, [1]],
+            [
+                [
+                    'alpha' => 'a',
+                    'bravo' => 'b',
+                    'charlie' => 'c',
+                ],
+                'b',
+                false,
+                ['bravo'],
+            ],
+            [
+                [
+                    'zero' => 0,
+                    'one' => 1,
+                    'true' => true,
+                ],
+                '1',
+                false,
+                ['one', 'true'],
+            ],
+            [
+                [
+                    'zero' => 0,
+                    'one' => 1,
+                    'true' => true,
+                ],
+                true,
+                false,
+                ['one', 'true'],
+            ],
+            [
+                [
+                    'zero' => 0,
+                    'one' => 1,
+                    'true' => true,
+                ],
+                true,
+                true,
+                ['true'],
+            ],
+            [
+                [
+                    'zero' => 0,
+                    'one' => 1,
+                    'true' => true,
+                ],
+                1,
+                true,
+                ['one'],
+            ],
+        ];
+    }
 }