Get current session in WP admin

Trying to access sessions in the WP admin and access a variable from it.

function tasks_admin_sessions() {
    if(!session_id()) {
        session_start();
    }
}
add_action( 'admin_init', 'tasks_admin_sessions', 1 );

$nonce = $_session['wp_nonce']
$completed_url = admin_url( "post.php?post=$post_idaction=trash_wpnonce=$nonce" );

but I get:

Notice: Undefined variable: _session

What am I missing in my code to get the session variables?