wp_timezone_choice(): fix bug in timezone dropdown list creation

This fixes a bug where if the `timezone_string` would be set to timezone name which has since been deprecated, no option would be (pre-)selected in the generated dropdown list and when the form using the dropdown list would be submitted, the "old", originally saved value would be lost as the form would submit without a value being selected for the `timezone_string` field.

The fix is a little hacky: it basically checks ahead of generating the actual dropdown list whether the `$selected_zone` value would be recognized and set to "selected" and if not, verifies the value _is_ a valid value, but outdated timezone name value and if so, adds an extra dropdown entry to the top of the list with the original value and sets this value to "selected".

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

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

Note: there are no pre-existing tests at all for this method and adding a complete set of tests for this method is outside the scope of this ticket, so this fix does not contain any tests.

Author: jrfnl

src/wp-includes/functions.php

@@ -6381,8 +6381,10 @@ function wp_timezone_choice( $selected_zone, $locale = null ) {
 		$mo_loaded = true;
 	}
 
-	$zonen = array();
-	foreach ( timezone_identifiers_list() as $zone ) {
+	$tz_identifiers = timezone_identifiers_list();
+	$zonen          = array();
+
+	foreach ( $tz_identifiers as $zone ) {
 		$zone = explode( '/', $zone );
 		if ( ! in_array( $zone[0], $continents, true ) ) {
 			continue;
@@ -6417,6 +6419,13 @@ function wp_timezone_choice( $selected_zone, $locale = null ) {
 		$structure[] = '<option selected="selected" value="">' . __( 'Select a city' ) . '</option>';
 	}
 
+	// If this is a deprecated, but valid, timezone string, display it at the top of the list as-is.
+	if ( in_array( $selected_zone, $tz_identifiers, true ) === false
+		&& in_array( $selected_zone, timezone_identifiers_list( DateTimeZone::ALL_WITH_BC ), true )
+	) {
+		$structure[] = '<option selected="selected" value="' . esc_attr( $selected_zone ) . '">' . esc_html( $selected_zone ) . '</option>';
+	}
+
 	foreach ( $zonen as $key => $zone ) {
 		// Build value in an array to join later.
 		$value = array( $zone['continent'] );