Purpose Of PHP Sessions and Cookies and Their Differences

I am just starting to learn to program in PHP and have ran into a slightly confusing area, Sessions and Cookies.

I understand the server-side and client-side storage differences but i cant see how they differentiate and in what circumstances would each be appropriate for?

Also, i have seen people say that the cookie could be used to store a session id, How would this be done and why would this be advantageous?

Thanks for any feedba开发者_C百科ck.

First of all, let's bust the longstanding myth (or at least I think it's an existing myth) that a session cookie is something different than a regular cookie. It is not. A session cookie is just a regular cookie. Only the properties of the session cookie that are set (or rather not set) are typically different. But the mechanism is exactly the same.

A cookie is set by sending a http response header to the browser:

Set-Cookie: name=value[; possible expiration-date][; other possible properties]

What typically distinguishes a session-cookie from a regular cookie is that no expiration date is set (or the expiration date is set to a date in the past). Which means the browser will dispose the cookie after closing the browser. But a 'regular' cookie can do this just as well. Thus thereby making it a 'session cookie' so to speak.

Now that we have that out of the way; the mechanism by which cookies are typically utilized by applications to make them act as even more of a session cookie, besides above mentioned properties, is that the value of the cookie only holds a uniquely identifiable value of some sort. Perhaps an md5 of maybe a sha1 hash.

Each time the browser requests a resource on the server it sends along this cookie (unless it has expired) with a http request header like this:

Cookie: name=value

The session mechanisms in the backend (being PHP in your case) linked the unique id of the cookie with data that has been stored in a file in the servers filesystem, or perhaps in a database. This way, each time the cookie is received it is able to retrieve this data and link it to the request.

The advantage of this, is that sensitive information 1) can be hidden from not having to travel over the net, and 2) doesn't end up in the users browser cookie cache, by keeping it at the server.

So, basically you want to send non-sensitive, and non-application-vital information in a regular cookie (think of: layout preferences, a non-persistant playlist such as on YouTube perhaps, etc.), and use a session to store sensitive information.

edit:
Sorry, ignore the "or the expiration date is set to a date in the past", as it was false. This will cause the cookie to immediately be invalidated by the browser, and thus not be sent along with requests anymore.

The advantage of using cookies over sessions is that cookies are persistent.

In other words, when the user visits your site weeks later, their session has more than likely expired. However, if they have a cookie that can uniquely identify them to your script, then you can automatically log them in and reestablish the session.

...what circumstances would each be appropriate for?

The answer looks something like this:

• Session data should contain information that does not need to be persistent or is only needed for a short period of time. For example, if you are presenting a multiple-page form to the user, it makes sense to take advantage of sessions.
• Cookies should be used to store an ID or hash that uniquely identifies not only the user, but also the browser / device they are logged in with. Keep in mind that cookie data is out of your control and can only be manipulated / removed by HTTP requests made by the user (or under certain circumstances, by a script on a page).

Also, i have seen people say that the cookie could be used to store a session id...

I'm assuming what was meant by that is storing a unique value in a cookie that identifies the user / browser / device that they are using. Implementing something like this would look like:

• Let the user log in as they would normally.
• Generate a unique hash (SHA-1 is your best bet) and store that in a cookie. You also store the hash in a database linked to that user.
• ...
• The user returns after their session has expired and visits a page.
• Your script sees the cookie and looks up the user that the hash belongs to.
• The user is logged in.

Both cookies and sessions are used to keep user-specific information in order to track a user. A lot of times you can use either one, but they have some differences.

A cookie is a text file kept on the user's machine. Every time the users visits your site he hands over the cookie letting you know who he is. The advantage of this is that the information is kept on somebody else's machine so you don't have to worry about it. As such you can leave it there until the cows come home. When/if the user comes back he'll bring the information with him. The downside is that the information is out of your control because the user can easily edit the cookie you gave him. This makes any information in a cookie untrustworthy and has to be checked every time the user gives it to you.

A session is like a cookie except you keep the information on your server. The advantage is that you can trust a session to keep data exactly like it was when you put it in. The downside is that you have to store that information which means that eventually you'll need to discard it lest your webserver fills up with information that will never be used.

Now this is where it gets a bit tricky. You see while the mechanism of a session is as I described above, the actual implementation can vary depending on PHP's settings. The session data can be kept in individual text files or in a database on your server. Also you need some way of recognizing which session corresponds to which user. The usual (but not only) way to do this is with cookies. What happens is that the actual data stays on your server and is linked to a unique session id. That session id number is put on a cookie and given to the user so you can later look up his data when he comes back.

The above process is performed automatically by PHP when you use the session functions; you do not need to implement it by hand. If for whatever reason you need to change how sessions are implemented you can do so by changing the session parameters in php.ini.