<?php

$answer = $_POST['answer'];

if($answer == '8'){

	echo json_encode(
		array(
			'success' => 1,
			'message' => 'Correct! 3 + 5 = 8'
		)
	);
	
	exit();

}else{

	echo json_encode(
		array(
			'success' => 0,
			'message' => 'Sorry, that is the wrong answer.'
		)
	);
	
	exit();

}


// just to have an extra failsafe!
echo json_encode(
	array(
		'message' => 'You must choose a valid response.'
	)
);

http://www.dropthenerd.com/ajax-a-simple-approach-that-you-will-use-and-reuse/

Example of a JSON encoded repsonse

json

PHP