You can’t.

PHP is a server side language. That means, whenever you request 
content from any website built with php, the server will send 
the rendered content i.e. HTML, CSS and JS, 
that means all the PHP code is firstly rendered to HTML 
and then it is sent to your browser, that is why you see only 
the HTML, CSS and JS whenever you view the source of any webpage.

That’s why you cannot see the PHP code for any website.