Is there an equivalent for PHP?
$Get_['asdf'] is one way to get user input... any others?
Nope, there is no equivalent. All php is executed on the server-side only. Unless you're using it at the command-line, which I doubt.
window.alert("Hi There, I am the Alert Box!")
For Prompt you can do something like this -
$prompt_msg = "Please type your name.";
$name = prompt($prompt_msg);
$output_msg = "Hello there ".$name."!";