If you're not familiar with the differences between the two (and there have been some good, concise posts illustrating that difference), i would first learn HTML, learn it well, and be clear on what it can and can't do. Once you understand that, then roll up your sleeves and dive into PHP; the reason being, if you're struggling to get things just to look right in HTML (which you will, if you're in the process of learning it), then imagine the terrible frustration you'll encounter when trying to make those elements interactive...because that's what PHP does.
Basically, PHP 'builds' an HTML web page on the server side. While you can 'View Source" in a web browser and see exactly how and why the page looks like it does, you'll never see the PHP behind a website, because by the time the page gets to you, PHP has already done it's job.
Furthermore, PHP is a great deal more complicated than HTML. Really, HTML is not a programming language...it's a set of tags that tell a browser how to display things, for instance, a green text box might look like this:
<div height="200" width="200" background="green">Some Text</div>
It's just formatting, plain and simple. PHP, on the other hand, is what you would use to make the page load "Some Text" automatically from a database, in the form of a news article, or a greeting to a specific user who has logged in.
These days, just about every site on the web uses a PHP backend, or something similar to it, so learning the language is a very worthwhile investment of your time.
Hope this helps.