Question: I always include the following DTD on my web pages:

```
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

```

I want to start using PHP on one of my sites. Should I modify this DTD in any way in order for it to remain valid?

And is there any reason why I should **not** use PHP blocks before the DTD or should the DTD always be the very start of each file?

Answer: 
PHP outputs HTML (or other browser displayable content). As long as the HTML that is output complies with your doctype, you can continue to use the doctype you've chosen. If you're not aware of what a doctype is and does, here is an old but good reference: <http://www.alistapart.com/articles/doctype/>
The choice of server-side technology used to generate pages does not affect the choice of doctype as such. What matters is the markup that gets sent to browsers.

The doctype statement (not a DTD – it’s in principle a reference to a DTD, though browsers don’t treat it that way) should be at the very start of an HTML document, but this applies to the HTML document sent to browsers. A .php file is not really an HTML document (or need not be), and any PHP instructions at the start are to be processed by server-side code and never reach a browser.