How to use many possible HTML pages that my PHP code can generate?

Each path through PHP code can generate a different HTML page.

I usually take several options for each success case and run them through the W3C HTML validator every release ... and at best I can hope to cover 20% of the pages that my code could generate.

This seems like a high order, but is there any tool that can at least reduce the amount of invalid HTML that my code can produce? Even just catching my beloved <p.instead <p>would be a boon.

I see how tough it would be ... even catching missing end tags can mean parsing in logic if ... elseor switchin PHP ...

I think I just answered my question ... but ...