title: Is it a good idea to choose php-session-id as primary key in a MySQL-Database?
body: <p>Is it a good idea to choose php-session-id as primary key in a MySQL-Database? Will there be unexpected surprises? </p>
  
  <p>Why I am asking this question: I want to store visitor-behavior and want to identify visitors on their session-id at that time.</p>
  
  <p>I do not know exactly what criteria a PHP session ID is generated. That's why I also do not know if someday might exist duplicates.</p>
  
  <p>EDIT2: Aside from performance issues a session ID is not always unique. Therefore, one should not use it as a PK. See here <a href="https://stackoverflow.com/questions/138670/how-unique-is-the-php-session-id">Link1</a></p>