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 is a server side language which generates text for the browser. The browser interprets HTML text exactly the same regardless of how the HTML was generated. Hard-coded html is interpreted in the browser the same as ASP-generated HTML, which is interpreted the same as PHP-generated HTML.

So your `Doctype` does **not** need to be customized based on any choice of server-side scripting language.
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/>