title: Create a UNIQUE 5 character alphanumeric string
body: <p>We are looking at creating promotional codes to send to customers, we have been told that each code sent HAS TO BE UNIQUE - 5 Characters - Alphanumeric.</p>
  
  <p>I thought of doing a hash of a concatenated string and taking the first 5 characters of the hash, but there is a good chance that the same 5 characters will come up again and again.</p>
  
  <p>Can anyone give me any pointers on creating this unique 5 character alpha numeric string that is unique EVERY TIME?</p>