What is the difference between PHP and HTML?
PHP is a server side programming language .
HTML is a mark up language .
PHP can do logical works like if certain conditions do this else do that . The o/p of PHP is markup 
. HTML works for static pages , PHP with HTML for dynamic pages .
with php you can connect to a database, read some records, execute operations etc, like any programming language. HTML is a custom XML formating language, understood by web browsers.
To put it simply:
PHP is a server-side language, HTML is a client-side language.
Other than that you need to really look it up! You need to study the general theory of programming.
One is 3 letters, the other 4… 
No really, did you do any search for an answer? Read a bit about HTML, and PHP? Then, if there is still something you want to ask, come back and ask