php的session不支持数字作为key的原因

php_serialize is available from PHP 5.5.4. php_serialize uses plain serialize/unserialize function internally and does not have limitations that php and php_binary have. Older serialize handlers cannot store numeric index nor string index contains special characters (| and !) in $_SESSION. Use php_serialize to avoid numeric index or special character errors at script shutdown.

参考： http://stackoverflow.com/questions/18797251/notice-unknown-skipping-numeric-key-1-in-unknown-on-line-0