Paul M Foster wrote:
This is in two parts. First cookies. I'm a little unclear on how they
work. From what I've read, cookies are stored by the browser. When a
request for that cookie comes in from the server, the browser returns
only the value, and no other data. One question: When the browser
requests a page from a server, does it automatically search its cookies
for that domain and send them along with the other requests? If that's
now how it works, then how does it work?

The browser sends all cookie information as part of the request.
You can see this by creating a dummy page that does not call seesion_start() that includes an XSS attack that reads the cookie and displays the session ID in an alert.

Second part is about sessions. According to the notes for the cookies
page at, it's considered bad practice to store user IDs and
passwords in cookies. It's considered better practice to use PHP's
native session-handling code to do this. But if a user has cookies
turned on in the browser, then PHP will store the session information
(possibly user ID and password) as a cookie. So what's the difference?

session stuff is stored server side, only the session ID is stored in the cookie.

It is still a bad idea to store username and password in the session because if you are on a shared server and you are not using a DB for session management (default is not to) another user on the server can read your cookies.

Even if you are using a database for session management, storing username/password in the session is a risk in case there is an sql injection attack that succesfully dumps your session database (which is bad enough w/o it exposing passwords).

I store a user id in the session and get the username from a db lookup if/when I need the username (but storing the username itself isn't really dangerous and would save an sql lookup in some cases).

There's no need to store password. If the user is not logged in, the session userid is set to 0. Anything that requires authentication in my code requires a session userid > 0 - and the userid can only be changed to a positive value via login.


A gotcha - changing a session variable doesn't actually happen until the script exits.

So if you set a session variable and then use the session variable later in the script, it will use the OLD value and not the new value, because the new value hasn't yet been written to the session.

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