populate_options(): fix bug in sanitization of localized default timezone_string

This fixes a bug where if the default `timezone_string` would be set to a deprecated timezone name due to a localization providing an outdated timezone name string, this localized timezone string would be discarded and an empty string would be set as the timezone value instead.

By passing the `DateTimeZone::ALL_WITH_BC` constant as the `$timezoneGroup` parameter to the PHP native `timezone_identifiers_list()` function, a timezone name list is retrieved containing both current and deprecated timezone names, preventing the invalidation of the option value.

See the extensive write-up about this in Trac#56468.

Also see: https://www.php.net/manual/en/datetimezone.listidentifiers.php

Includes an expansion of the translators comment to encourage translators to use "old" names over "new" names.

Includes adding a dedicated test to the `Tests_Admin_IncludesSchema` test class.

Author: jrfnl

src/wp-admin/includes/schema.php

@@ -388,13 +388,22 @@ function populate_options( array $options = array() ) {
 	/*
 	 * translators: default GMT offset or timezone string. Must be either a valid offset (-12 to 14)
 	 * or a valid timezone string (America/New_York). See https://www.php.net/manual/en/timezones.php
-	 * for all timezone strings supported by PHP.
+	 * for all timezone strings currently supported by PHP.
+	 *
+	 * Important: When a previous timezone string, like `Europe/Kiev`, has been superseded by an
+	 * updated one, like `Europe/Kyiv`, as a rule of thumb, the **old** timezone name should be used
+	 * in the "translation" to allow for the default timezone setting to be PHP cross-version compatible,
+	 * as old timezone names will be recognized in new PHP versions, while new timezone names can not
+	 * be recognized in old PHP versions.
+	 * To verify which timezone strings are available in the _oldest_ PHP version supported, you can
+	 * use https://3v4l.org/6YQAt#v5.6.20 and replace the "BR" (Brazil) in the code line with the
+	 * country code for which you want to look up the supported timezone names.
 	 */
 	$offset_or_tz = _x( '0', 'default GMT offset or timezone string' );
 	if ( is_numeric( $offset_or_tz ) ) {
 		$gmt_offset = $offset_or_tz;
-	} elseif ( $offset_or_tz && in_array( $offset_or_tz, timezone_identifiers_list(), true ) ) {
-			$timezone_string = $offset_or_tz;
+	} elseif ( $offset_or_tz && in_array( $offset_or_tz, timezone_identifiers_list( DateTimeZone::ALL_WITH_BC ), true ) ) {
+		$timezone_string = $offset_or_tz;
 	}
 
 	$defaults = array(

tests/phpunit/tests/admin/includesSchema.php

@@ -177,6 +177,47 @@ public function data_populate_options() {
 		);
 	}
 
+	/**
+	 * Safeguards that deprecated timezone strings set as a default in a translation are handled correctly.
+	 *
+	 * @ticket 56468
+	 */
+	public function test_populate_options_when_locale_uses_deprecated_timezone_string() {
+		global $wpdb;
+
+		// Back up.
+		$orig_options  = $wpdb->options;
+		$wpdb->options = self::$options;
+
+		// Ensure the "default" value for the timezone will be set to a deprecated timezone.
+		add_filter(
+			'gettext_with_context',
+			function( $translation, $text, $context ) {
+				if ( '0' === $text && 'default GMT offset or timezone string' === $context ) {
+					return 'America/Buenos_Aires';
+				}
+
+				return $translation;
+			},
+			10,
+			3
+		);
+
+		// Test.
+		populate_options();
+
+		wp_cache_delete( 'alloptions', 'options' );
+
+		$result = get_option( 'timezone_string' );
+
+		// Reset.
+		$wpdb->query( "TRUNCATE TABLE {$wpdb->options}" );
+		$wpdb->options = $orig_options;
+
+		// Assert.
+		$this->assertSame( 'America/Buenos_Aires', $result );
+	}
+
 	/**
 	 * @ticket 44896
 	 * @group multisite