Use sel as opposed to the class name

[darrenwh]

Can I suggest that self:: be used instead of IpnListener:: lines 170 and 171

http://php.net/manual/en/language.oop5.constants.php

private function getPaypalHost() {
    if ($this->use_sandbox) return self::SANDBOX_HOST;
    else return self::PAYPAL_HOST;
}

[MicahCarrick]

Agreed and done.