Update samples/DirectPayment/DoDirectPayment.php

Missing PaymentAction
Without this option, all transactions are treated as Sale
Merge pull request #5 from diemuzi/patch-1

Update samples/DirectPayment/DoDirectPayment.php
Updated ipn sandbox endpoint
is_array condition check interchanged
Update lib/PPCredentialManager.php

MissingCredentialException() does not exist
Replaced with PPMissingCredentialException
Merge pull request #7 from diemuzi/patch-2

Update lib/PPCredentialManager.php
Merge remote branch 'origin/master' into correct-require