From looking at the Validator response and Viewing Source, I can see a few things straight away. These are (X)HTML issues (which is what the Validator is checking out) not PHP. The Validator checks the page that is viewed by the Browser (Client-side) after PHP has done it's dynamic trickery (Server-side).
1. In your template, you have
<h5> tags used as content containers - change these to
<div> with different classes for your style hooks. That will take care of all the warnings about
<p> in invalid locations.
2. Your Digg and Del.icio.us links have unescaped Ampersands;
& should be coded as
p;. This may be something you can edit in your post template or may be the result of a plugin. How are you generating these links?
<br> tags are present. This is an XHTML document (according to DOCTYPE!) so these should be
4. In the footer, the "x calls to the database" line is lacking a closing
That's a few things off the top of my head, there may be more. Can I recommend that you run your templates through HTML Tidy (or similar) before uploading and, in any case, Validate, Validate, Validate.