Pings/Trackbacks: Validate that the specified charset is available on the receiving site.

This aims to avoid a subsequent fatal error from `mb_convert_encoding()` when an invalid charset is specified.

Follow-up to [1734], [2563], [12032].

Props dd32, jrf, oglekler, rajinsharwar.
Fixes #60261.

git-svn-id: https://develop.svn.wordpress.org/trunk@59255 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-trackback.php

@@ -60,7 +60,14 @@ function trackback_response( $error = 0, $error_message = '' ) {
 
 if ( $charset ) {
 	$charset = str_replace( array( ',', ' ' ), '', strtoupper( trim( $charset ) ) );
-} else {
+
+	// Validate the specified "sender" charset is available on the receiving site.
+	if ( function_exists( 'mb_list_encodings' ) && ! in_array( $charset, mb_list_encodings(), true ) ) {
+		$charset = '';
+	}
+}
+
+if ( ! $charset ) {
 	$charset = 'ASCII, UTF-8, ISO-8859-1, JIS, EUC-JP, SJIS';
 }