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.