Is this session handler secure?
Nope. Let's walk through it from a hypothetical attackers point of view.


  Okay. It's sending an identification cookie. It's hex. 32 digits long.
  Probably an md5 hash. I wonder what's hashed? I'll throw it in
  oclHashcat to check. I can make a billion guesses a second, so if it's
  a hash of something simple, I'll find out.
  
  &lt;3 seconds later>
  
  Huh. It's a number. Lemme grab another session and see if that's a
  number too.
  
  Yep.
  
  It looks like these are all 6 digit numbers. I'll grab 10 more to
  check. 
  
  Okay, 6 digits AND they can be positive or negative.
  
  That's a keyspace of about 2^21. If there are 8 sessions going right
  now, then it costs, on average,
  
  2^(21 - 1 - 3) requests * 1 KB per request * 0.08 $/GB
  ...
  =1.01 cents to crack a session.
  Alright, I'll start on that and see what I have in the morning.


So: Add more entropy.