Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 22136)
+++ wp-includes/functions.php	(working copy)
@@ -252,6 +252,9 @@
  	if ( 'N;' == $data )
 		return true;
 	$length = strlen( $data );
+	// if mbstring.func_overload is enabled, strlen() may not return the full length in bytes for UTF-8 strings
+	while ( isset( $data[ $length ] ) )
+		$length++;
 	if ( $length < 4 )
 		return false;
 	if ( ':' !== $data[1] )
@@ -289,6 +292,9 @@
 		return false;
 	$data = trim( $data );
 	$length = strlen( $data );
+	// if mbstring.func_overload is enabled, strlen() may not return the full length in bytes for UTF-8 strings
+	while ( isset( $data[ $length ] ) )
+		$length++;
 	if ( $length < 4 )
 		return false;
 	elseif ( ':' !== $data[1] )