What is the difference between session_create_id and session_regenerate_id()?

Question

Doc says

session_create_id() is used to create new session id for the current session. session_regenerate_id() Update the current session id wi

Answer 1

session_create_id

Create new session id

session_regenerate_id

Update the current session id with a newly generated one

Usage example from the manual:

$old_sessionid = session_id();

// Set destroyed timestamp
$_SESSION['destroyed'] = time(); // Since PHP 7.0.0 and up, session_regenerate_id() saves old session data

// Simply calling session_regenerate_id() may result in lost session, etc.
// See next example.
session_regenerate_id();

// New session does not need destroyed timestamp
unset($_SESSION['destroyed']);

$new_sessionid = session_id();

echo "Old Session: $old_sessionid
";
echo "New Session: $new_sessionid
";

print_r($_SESSION);

Which leads us to the following question - why and when you should use it, there's a detailed answer in this link.