Update example code for using-cas.php

The code https://github.com/couchbase/docs-sdk-php/blob/release/3.1/modules/howtos/examples/using-cas.php doesn't run.

In particular:

• the credentials/bucket configuration don't match what's created in e.g the Docker server-sandbox
• function flow isn't wired up properly (e.g. replaceWithCas isn't defined)

NB: as a result, I've recreated some of the logic in the errors example https://github.com/couchbase/docs-sdk-php/pull/56/commits/4dd53f3ca38f5bd3897e5f01313352e325410ed8#diff-be86df73944a21f7a1e652eae7c0a703fc738c405971d935b186a1377f8e0222R40-R59 where it might have been more elegant to link to a snippet here in future.