Generating Custom Session ID in PHP 5

Another one of my favorite questions from Zend PHP 5 Certification Mock Exam. Correct answer in bold.

If you would like to change the session ID generation function, which of the following is the best approach for PHP 5?

• Set the session.hash_function INI configuration directive
• Use the session_set_id_generator() function
• Set the session id by force using the session_id() function
• Use the session_regenerate_id() function
• Implement a custom session handler

Surprisingly, the most obvious answer, made sound like something not really reasonable, is the correct one. A few words of explanation.

• session.hash_function allows to choose between MD5 and SHA-1 hash algorithm for session id generation
  
• session_set_id_generator() function does not exist, at least according to session handling chapter of PHP documentation
• session_regenerate_id() function uses the algorithm selected by session.hash_function
• custom session handler is not meant to allow setting functionality to generate session ids

And session_id() is the only one on the list allowing to set custom session id and definitely it is the correct answer.