Simple public Web API in PHP using JSONP

JSONP_Example.php

<?php
/* JSONP_Example.php
 * Author: Kevin Lanni
 * Description: Demonstrates how to create a simple public Web API using JSONP responses
 *		This is useful for opening Web APIs for public use without the need for proxying or other SOP work-arounds.
 *		This example does not demonstrate API keys or any other method of authentication.
 */

// Supply a header to set the proper expectation for the client browser
header('Content-Type: application/json');

// A simple error handler, so we can output the error to the requesting script instead of failing silently
function errHandler($errno, $errstr) {
	switch ($errno) {
		// You can specify specific response data depending on error type, but for this example we'll only set a default
		default:
			$result = Array(
				"success" => false,
				"error" => $errstr,
				"params" => $_GET
			);
			
			if (isset($_GET["callback"])) {
				echo $_GET["callback"] .'('. json_encode($result) .');';
			} else {
				echo json_encode($result);
			}
			
			exit;
		break;
	}
}

// Set our errHandler function above to be the default error handler for PHP
set_error_handler("errHandler");

//
// Here you would put your PHP code that handles the API requests and builds responses.
//

// What I like to do is set a variable such as "$result" to an assoc array for my JSON data, like so:

$result = array(
	"success" => true,
	"message" => "You have successfully implemented a simple Web API using JSONP!"
);

// Then I would convert it to JSON using json_encode at the end of the script, when I'm ready to generate the output.

// For error handling, I use trigger_error, which is as simple as:

// if ($some_condition == "unmet") {
	// trigger_error("Some condition was not met!", E_USER_ERROR); // Where E_USER_ERROR can be any valid PHP error constant (http://www.php.net/manual/en/errorfunc.constants.php)
// }
// The script would stop executing as soon as the first error is detected, because we exit the script in the error handler. This helps prevent lockups.
// Of course, you may want to handle warnings less strictly and do something else for your warnings. See error handler.
// Uncomment this if-statement to see an error thrown for "Undefined variable: some_condition". Uncomment just the trigger_error statement to see a custom error message.

// Once we're done executing all the necessary code to generate our data, let's convert it to JSON and print it out
if (isset($_GET["callback"])) {
	echo $_GET["callback"] .'('. json_encode($result) .');';
} else {
	// Just in case we're not making cross domain requests, we can forego the JSONP and just expect regular JSON
	echo json_encode($result);
}
?>