DHTML - Client side language..you can not create forums or login systems or connect to database using HTMl because everything is done at client side i.e. browser..
PHP - It is server side language..it is used to create web applications like shopping carts, creating images (GD Library), forums...basically anything you can think of...
how does it work?
well its like...
php will query the server for data and then after fetching the data it will convert the same in HTML or whatever format you see..
for example...
to get data from this post...PHP may query mysql like foll
Code:
$sql="SELECT * FROM posts where post_id=6896";
$result = mysql_query($sql);
$fetch_data = mysql_fetch_row($result);
// rest will come here..
so basically this query is just asking databse to get records for specified post id...
and once it gets all details it i wll place all in html format..
so what you see in the source code is simple html not the code which is written to make html (i.e. the code i mentioned before)
i hope you got the point...
so in short if you want to learn php, learn html first because its the base for all web languages...
Regards
Deep