On many places (Google, Yahoo, Stack Exchange... ) I found that Ajax coding (for example php+Ajax login system) is not, and cannot be secure enough. So many reserve about Ajax security, and nowhere you can find an example of secure Ajax code.
At the same time, all this sites (even Facebook, Twitter...) USES a lot of Ajax code, for registering and loging users, for commenting features etc. Seems like Top Secret matter.
So, could someone show an example of secure php-Ajax code ?