WL#3436: PHP: ext/mysql -> ext/mysqli converter tool -- article: API differences ext/mysql and ext/mysqli

Write an article which describes in detail the API differences between ext/mysql
and ext/mysqli. The article should help people understanding a) the compromises
the converter tool has to make b) how to rewrite the generated code manually to
be optimized for the target application.

It's planned to use an article series instead of classical documentation to
describe the tool. The advantage of articles is that we can them for marketing
purposes. Classical documentation seems not needed as the usage of the tool
itself has to be made self-explaining.