Ya, you'll need to upgrade your PHP. PHP 5.3 had been EOL for several years now. If possible, upgrade to PHP 7 (7.0, 7.1 or 7.2). Don't upgrade to 7.3 as XenForo is not PHP 7.3 ready. If you can't upgrade to PHP 7, at least upgrade to PHP 5.6, altho, IIRC, that error will not be thrown if using PHP 5.5.