I wouldn't say it is impossible. there are several frameworks that start you out on a fast track. Now if you say your are going to build a facebook like site with the same breadth, power and traffic capabilities. No my man. No. Not without a lot of help and dollar bills.
Free PHP Proxy Network You should try googling tutorials for $_GET and then you might finally know how difficult it is to get a "twitter" like website going.
It's OK getting the server to handle the same traffic capabilities as FB but there's no way anybodys actually going to get that traffic
w3schools is ideal if PHP is new to you but would not rely on it for your project. I found the mysql site as well as php.net and phptutorial.info sites useful