Correct Session Authentication

[xbennx () hotmail co uk]

I've been developing a shopping cart for my friends company and have just started using sessions to authenticate 
people. After reading many tutorials on the internet about sessions and PHP I coded a login page. I keep hearing about 
session id's but all the tutorials I read didn't mention them.

When a user logs on, the username and password are sent via SSL and the md5 hash is then checked against a hash stored 
in database. If the credentials are found in the database, the users id is return and stored in a session. If the 
credentials are not found this session value is 0. Every page that a user needs to be authenticated to see checks to 
see that the user id stored in the session is greater than 0, if not the user is classed as not authorised as 0 is not 
a valid user id.

Is this method secure or can it be easily bypassed? 

Another thing I was wondering is where are sessions values actually stored? I've read that they're stored in cookies 
but I always thought there was a seperate function in php to create cookies?

Sorry this is so long, any help will be much appreciated.

Thanks

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