Question:

HTML

<p id="a" name="a">

Javascript

document.getElementById("a").innerHTML=price;

If price is a variable how do I make a php variable that is = to the result that is placed in the HTML of id "a" i?

$price = $_POST['a'];

when echoed this variable comes up blank, and yes I do have a form set up that is handled by a PHP page.

That's not how forms work.

The browser will only send values of <strong>form controls</strong> (<input>, <select>, <textarea>) to the server.<br /> It won't send values of arbitrary named elements. (in fact, that name="a" is invalid HTML)

Instead, you should use an <input type="hidden" name="a" value="" />, and set the value attribute of this <input> tag to the value you want PHP to get.

I guess, you want only to change the values in HTML elements. For that you need a XML parser (<a href="http://www.php.net/manual/en/book.xml.php" rel="nofollow">i.d</a>). I think <a href="https://stackoverflow.com/a/3627553/1493365" rel="nofollow">shamittimar's post</a> may help you. He does exactly what you want.