Why php session does not support the number of objects as the key

Php session does not support numbers as keys. Why does php session not support numbers as keys?

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.

Reference :? Http://stackoverflow.com/questions/18797251/notice-unknown-skipping-numeric-key-1-in-unknown-on-line-0