Add this to you functions.php
add_action('init', 'myStartSession', 1); //Initialize session
add_action('wp_logout', 'myEndSession'); //End session on logout
add_action('wp_login', 'myEndSession'); //End session on account change
function myStartSession() {
if(!session_id()) {
session_start();
}
}
function myEndSession() {
session_destroy ();
}
Access the value in normal PHP fashion
$_SESSION['myKey'] = "Some data I need later";
Source: https://silvermapleweb.com/using-the-php-session-in-wordpress/