Functions don't need parenthesis in phpdoc.
Before making any changes to post content or outputting any information gleaned from the post content, check if a password is required and has been provided if needed.

See https://github.com/ethitter/WP-Print-Friendly/pull/4.
Convert plugin to a singleton pattern, aliasing former global variable `$wpf` in case users are referencing it.
Correct phpdoc implementation