Yup... like he said, PHP is pretty useless without HTML. You can think of HTML as the GUI for PHP. PHP does things like handle forms, interact with databases, etc. - it's a programming language, whereas HTML is just a mark-up language that throws together visual elements. For example, HTML allows to put the form elements on the page, change the background color, insert images, etc. but PHP takes the information inserted into the form and does something with it like send an email or put it in a database.
There's a plethora of PHP resources compiled in
this thread. (Speaking of which, this thread belongs in web development... I'll move it...)