Hi Evans,
PHP is different to HTML and it has a different structure and appearance, so unless you have learned to recognise the tags, you won't understand them. In any case, PHP is used to add function to your code, not structure it for output (which is what HTML is used for) per se so you wouldn't really be able to "see" it just by looking at a web page. You would only be able to see it working when you run the PHP scripts on that page.
There are plenty of free resources on the net to get you started, if you want to know more. I would suggest W3Schools for some of the basics.
The problem that you can't preview the files is because PHP it's a server-side language and needs a server to run. Dreamweaver is a client-side app, so it would not be able to interpret the code as far as I know. You would either need to upload your PHP to the net and check that it works in a browser or use an IDE with a server-like live preview function. I haven't worked with Dreamweaver for a while, so I don't know if they have built one in. Programs like PHPEdit and WeBuilder have this function, but they are intended for hand coding.
Anyway, I hope that sheds some light on the matter, not that it would really help much I think.