Most times, PHP runs on a web server out there on the internet somewhere. Buttons are defined when your browser loads HTML code like the code you just posted.
If you want some PHP code to run on the server, then your form looks like it'll do that. You just need to put the code you want to run in the get.php file on your server.
If you want PHP to run via Command Line Interface (CLI) then you could put something like this in get.php:
$last_line_of_output = exec("ls -l");
Not every server is configured to allow exec to run.