Excerpt from http://php.about.com/od/advancedphp/ss/php_sessions.htm:
So how will it know it's me? Most sessions set a cookie on your computer to uses as a key... it will look something like this: 350401be75bbb0fafd3d912a1a1d5e54.
My question is, in PHP, how to generate a key (e.g., 350401be75bbb0fafd3d912a1a1d5e54) for a session cookie?
And when do we need such a key? Why not just set $_SESSION['color']='red' in the first page and retrieve in the second page with $_SESSION['color']?
how to generate a key (e.g., 350401be75bbb0fafd3d912a1a1d5e54) for a session cookie?
Just call session_start() for this. A key would be generated automatically
when do we need such a key?
when session starts, to distinguish one user from another
Why not just set $_SESSION['color']='red' in the first page and retrieve in the second page with $_SESSION['color']?
This is the way sessions works. You are encouraged to do it this way. Who says you can't do it?
When youu start a session in PHP using session_start it auto generates a session key.
Check the session section on the PHP manual http://www.php.net/manual/en/book.session.php
And when do we need such a key? Why not just set $_SESSION['color']='red'
in the first page and retrieve in the second page with $_SESSION['color']?
The key's a unique identifier for each user to your site. If everyone received the same session ID, then they'd all be sharing the same session ID. Think of what'd happen if your bank's website used the same key for everyone. The first person to log in would then have their account exposed to every other visitor.
You can store whatever you want in the $_SESSION array, but remember that if things were correctly configured, it's going to be a different array for every user, so only store whatever's "configurable" per-user. A color preference for a background, like your 'red' example is one. But don't store the name of your site, as that wouldn't differ for each user.