To create a session only cookie, you need to set the expiration date of the cookie to be some years ahead. You need to set the expire attribute:
current time + 5 years
The expire attribute shows the date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser.
Note: Do not consider removing the expire value to create a session only cookie. This won’t work. Set the expire attribute to the future date.