Single-routine PHP session handling

sess.php

define("SESSION_FILE", "/tmp/session.txt");
$session = [];

function sess($k, $v = NULL) {
        global $session;

        if($session == NULL) {
                if(!file_exists(SESSION_FILE))
                        touch(SESSION_FILE);
                $session = file_get_contents(SESSION_FILE);
                $session = $session ? unserialize($session) : [];
                register_shutdown_function(function() {
                        global $session;

                        file_put_contents(SESSION_FILE, serialize($session));
                });
        }
        $t = @$session[$k];
        if($v != NULL)
                $session[$k] = $v;
        return $t;
}