Why is Facebook PHP SDK getUser always returning 0?

I had the same problem and I figured it out that is because SDK uses the variable $_REQUEST and in my environment is not true that is merged with $_GET, $_POST and $_COOKIE variables.

I think it depends on the PHP version and that is why someone made it work by enabling cookies.

I found this code in base_facebook.php:

protected function getCode() {
    if (isset($_REQUEST['code'])) {
        if ($this->state !== null &&
                isset($_REQUEST['state']) &&
                $this->state === $_REQUEST['state']) {

            // CSRF state has done its job, so clear it
            $this->state = null;
            $this->clearPersistentData('state');
            return $_REQUEST['code'];
        } else {
            self::errorLog('CSRF state token does not match one provided.');
            return false;
        }
    }

    return false;
}

And I modified it as you can see below by creating $server_info variable.

protected function getCode() {
    $server_info = array_merge($_GET, $_POST, $_COOKIE);

    if (isset($server_info['code'])) {
        if ($this->state !== null &&
                isset($server_info['state']) &&
                $this->state === $server_info['state']) {

            // CSRF state has done its job, so clear it
            $this->state = null;
            $this->clearPersistentData('state');
            return $server_info['code'];
        } else {
            self::errorLog('CSRF state token does not match one provided.');
            return false;
        }
    }

    return false;
}