Close session and start a new one

I'm testing the implementation of a security check in my PHP sessions. I can successfuly detect whether the session was started from another IP address and I can successfully start a new session. However, the data from the old session gets copied into the new one! How can I start a blank session while preserving the previous session data for its legitimate owner?

This is my code so far, after lots of failed attempts:

<?php

// Security check
if( isset($_SESSION['ip_address']) && $_SERVER['REMOTE_ADDR']!=$_SESSION['ip_address'] ){
    // Check failed: we'll start a brand new session
    session_regenerate_id(FALSE);
    $tmp = session_id();
    session_write_close();
    unset($_SESSION);
    session_id($tmp);
    session_start();
}

// First time here
if( !isset($_SESSION['ip_address']) ){
    $_SESSION['ip_address'] = $_SERVER['REMOTE_ADDR'];
    $_SESSION['start_date'] = new DateTime;
}

The official documentation about sessions is terribly confusing :(

Update: I'm posting some findings I got through trial and error. They seem to work:

<?php

// Load the session that we will eventually discard
session_start();

// We can only generate a new ID from an open session
session_regenerate_id();

// We store the ID because it gets lost when closing the session
$tmp = session_id();

// Close session (doesn't destroy data: $_SESSION and file remains)
session_destroy();

// Set new ID for the next session
session_id($tmp);
unset($tmp);

// Start session (uses new ID, removes values from $_SESSION and loads the new ones if applicable)
session_start();