title: Credit Card - How Unique is First6,Last4, expMonth & expYear
body: <p>I'm building a manager class with PHP to manage credit card payment authorizations. With credit cards, we're allowed to keep <code>First6</code>, <code>last4</code>, <code>expiration_Month</code> and <code>expiration_Year</code>. </p>
  
  <p>I'm really interested in knowing how unique the combination of these 4 variables is and how likely it would be to run into another one.  </p>
  
  <p>Depending on how likely it is will effect when to test if we've already got a valid authorization for a new card. If we've already got an authorization for a particular card, there's no need to run the numbers again. Instead, we can find the already authorized card and do a re-authorization. However, I wouldn't want to run the wrong card because it has a similar <code>First6</code>, <code>last4</code>, <code>expiration_Month</code> and <code>expiration_Year</code>.. </p>
  
  <p>My goal is to limit data redundancy of credit card data, hits to the CC processor API and unnecessary authorizations on customer cards.</p>