What are you trying to do? I'm a PHP tinkerer at best, with a lot of help, but I've been using PHP in WordPress for about a decade.
If you're talking about creating a plugin, that's a process that's pretty easy to start, but really tying it into your site, you'll need to study up on WordPress' codex and template tags:
https://codex.wordpress.org/Template_Tags
If you're talking about putting information onto certain pages, now you're talking about inserting PHP into a page, which isn't native in WordPress. I'd get a plugin called PHP Code Snippet by XYZscripts. It's fantastic, and it allows you to easily use code snippets through your site. From there, I use INCLUDES to associate PHP files to my WP Pages, and I update the PHP files in any text editor.
I would recommend checking out the Template Tags no matter what. You can use them in your PHP files just for simple calls, as a lot of what I do involves referring to items within WP.