Form validation tools

In the Simplified Form Errors comments, J. Shell mentions the use of a form validation toolkit.

I’ve evaluated a number of form kits and validators over the years for PHP, Java, ASP, and Perl. They all have fallen short of my expectations and the needs of real-world development. Some of them have faulty validation — letting things through that shouldn’t be or disallowing valid input. Others aren’t flexible enough to allow non-standard inputs. Sometimes I need a rather custom validation like 32 characters, numbers and lowercase letters only, no spaces. And many packages won’t let you inject your own error messages into the error output. For example, just because a username validates doesn’t mean someone can register it. There may already be someone using that name, so I’d need to insert an error that the user needs to pick a different name.

I’d certainly be open to a good form validator, either commercial or open source. Since I’ve recently been doing most of my development in PHP, a PHP form toolkit would be most useful. Any suggestions?