PHP is a dynamic language meaning it can send, get and display data. Its used for server request mostly for example when you click on a button to search on google it send a request to a php script...."yo send me info on HTML!" the php script will then look up all the info it can find in the sites database(mySql) and send it back to you. thats just one way php is used. its used mostly the get info, talk to other scripts and disply info on your screen.
(you can even build game in PHP but thats just stupid)
HTML is a building language it inserts text on a page gives text colors, puts back ground images on a site. It builds
Now if your asking which one should you learn. you need to know HTML be for you can do anything. HTML the only foundation there is to make a site. you cant really have a php code on a page by it self(well you can but thats not for you right now) every page has to start with a HTML tag and close with a HTML closing tag. So there is no Vs. They both are needed
P.S. (Google does not use php they use python, its like PHP but better)
got it? -=)