It does not. Your browser may behave differently, you may behave differently. But the PHP code does not depend on the browser. It cannot. It's impossible. The server doesn't know at all which browser you're using. You say you understand the concept, but you keep repeating this wrong idea.
Originally Posted by we5inelgr
Think of client and server as two people in two different rooms separated by a big wall. They can't see each other, and they can't hear each other. There's only a small slot they can use to exchange messages written on a paper. Now, is it possible that the server behaves differently depending on whether the client is male, female, black, white, ...? No! The server knows nothing about the client except where he/she is. The server may act depending on the message or just do random stuff. And it may even look like, say, male clients and female clients are treated differently. But it's impossible. Either the messages are different, or it's just chance.
OK, at least we know now that both browsers do send cookies. So the question is: What happens between storing the session values and using them?
Make two scripts:
$_SESSION['foo'] = 'bar';
echo 'Writing session<br />';
echo 'Session ' . session_id() . ' should contain the following:<br />';
Delete all existing session cookies in both browsers and clear the session folder (unless there's something important in it). Visit the "write" script with both browsers and then check the cookie and the session file: Did both browsers store the session cookie? Do both sessions exist on the server and have the right content? Then visit the "read" script with both browsers: Do both browsers send the original session cookie? What's the actual session ID as used by PHP?
echo 'Session ID sent with cookie:<br />' . $_COOKIE['PHPSESSID'] . '<br />';
echo 'Actual session ID used by PHP:<br />' . session_id() . '<br />';
echo 'Content of session:<br />';
Post the answers and the output of both scripts here.