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.
In brief
--------

The `DOCTYPE` has nothing to do with PHP, it is what is returned to the browser that you need to be aware of.

Detailed Answer
---------------

The `DOCTYPE` simply tells the browser how to interpret the page and the code that makes up the page. As `PHP` is a server side language, it has nothing to do with this, unless of course you are outputting specific HTML that requires a different `DOCTYPE` to the one you have defined.

Credit
------

@adeneo has rightly added that you should alter it, but not because it has anything to do with PHP. For HTML 5 you must declare the doctype using:

```
<!DOCTYPE html>

```