Love the idea and I think it's a great resource to centralize most common solution like using phpass to hash/salt passwords to avoid people reinventing the wheel all the time.

Could you elaborate on why you chose PDO over MySQLi to interact with MySQL? Especially with all the "specific things to do for it to work properly" at the end of the section?

The email address validation is also a great section, everyone tries to rewrite its own and fails or forgets to handle cases (such as '+' in first part). I think it would be great to show other filters examples instead of providing a link to the documentation, like how to sanitize a username entry for example?

Another interesting section would be how to handle errors properly, like the use of trigger_error(), set_error_handler() or exceptions. Seems that a lot of people are also doing their own logging/debugging/reporting system not knowing where to start?