It’s a very bad idea to modify WP Core files for the ability to use sessions. The best way I’ve found is to call the session_start()
from init
action hook.
function kana_init_session() {
session_start();
}
add_action('init', 'kana_init_session', 1);
You can place it in functions.php
file of your theme.
Detailed article can be found here: http://www.kanasolution.com/2011/01/session-variable-in-wordpress/