Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 15646)
+++ wp-includes/functions.php	(working copy)
@@ -228,15 +228,18 @@
  * @return bool False if not serialized and true if it was.
  */
 function is_serialized( $data ) {
+	static $strpbrk_exists;
 	// if it isn't a string, it isn't serialized
 	if ( !is_string( $data ) )
 		return false;
 	$data = trim( $data );
  	if ( 'N;' == $data )
 		return true;
-	if ( function_exists('strpbrk') ) {
-		if ( strlen($data) > 1 && strpbrk($data,'adObis') == $data && $data[1] == ':' ) {
-			$badions = array();
+	if ( strlen( $data ) < 4 )
+		return false;
+	isset( $strpbrk_exists ) || $strpbrk_exists = function_exists( 'strpbrk' );
+	if ( $strpbrk_exists ) {
+		if ( strpbrk( $data, 'adObis' ) === $data && ':' === $data[1] ) {
 			$badions[1] = $data[0];
 		} else {
 			return false;