The short answer is you can't because PHP is a server-side code and doesn't get sent to the browser, the php tells the server what to send to the browser.
The long answer is you can by hacking the site which isn't really a nice thing to do so I won't go into details, plus lots of people might show you the index.php file and the index file will have lots of if statements with include() calls to files which are in a folder that only the server can access so even if you do hack the site you're not going to get any decent code, probably just the initial page.
The long answer is you can by hacking the site which isn't really a nice thing to do so I won't go into details, plus lots of people might show you the index.php file and the index file will have lots of if statements with include() calls to files which are in a folder that only the server can access so even if you do hack the site you're not going to get any decent code, probably just the initial page.