Hey there, I recently tried to get some info out of my Zabbix instance to use in another context and therefore had a look at Zabbix’ API.
Turns out, it is quite simple to use and works with JSON messages.

Communication flow is pretty simple:

1. Send username and password to the API
2. Retrieve Auth-Token from API
3. Send your actual query to the API and append the Auth-Token
4. Retrieve queried data from API

To test communication, I wrote a simple PHP script to fiddle around with the possibilities of the API:

<?php

/* 
          _     _     _      
 ______ _| |__ | |__ (_)_  __
|_  / _` | '_ \| '_ \| \ \/ /
 / / (_| | |_) | |_) | |>  < 
/___\__,_|_.__/|_.__/|_/_/\_\  - API PoC

2012, looke

*/

$uri = "https://zabbix.foo.bar/api_jsonrpc.php";
$username = "testuser";
$password = "xyz";

function expand_arr($array) {	
	foreach ($array as $key => $value) {
		if (is_array($value)) {			
			echo "<i>".$key."</i>:<br>";
			expand_arr($value);
			echo "<br>\n";
		} else {			
			echo "<i>".$key."</i>: ".$value."<br>\n";
		}		
	}
}

function json_request($uri, $data) {
	$json_data = json_encode($data);	
	$c = curl_init();
	curl_setopt($c, CURLOPT_URL, $uri);
	curl_setopt($c, CURLOPT_CUSTOMREQUEST, "POST");                                                  
	curl_setopt($c, CURLOPT_RETURNTRANSFER, true); 
	curl_setopt($c, CURLOPT_POST, $json_data);
	curl_setopt($c, CURLOPT_POSTFIELDS, $json_data);
	curl_setopt($c, CURLOPT_HTTPHEADER, array(                                                                          
		'Content-Type: application/json',                                                                                
		'Content-Length: ' . strlen($json_data))                                                                       
	);
	curl_setopt($c, CURLOPT_SSL_VERIFYPEER, false);	
	$result = curl_exec($c);
	
	/* Uncomment to see some debug info
	echo "<b>JSON Request:</b><br>\n";
	echo $json_data."<br><br>\n";

	echo "<b>JSON Answer:</b><br>\n";
	echo $result."<br><br>\n";

	echo "<b>CURL Debug Info:</b><br>\n";
	$debug = curl_getinfo($c);
	echo expand_arr($debug)."<br><hr>\n";
	*/

	return json_decode($result, true);
}

function zabbix_auth($uri, $username, $password) {
	$data = array(
		'jsonrpc' => "2.0",
		'method' => "user.authenticate",
		'params' => array(
			'user' => $username,
			'password' => $password
		),
		'id' => "1"
	);	
	$response = json_request($uri, $data);	
	return $response['result'];
}

function zabbix_get_hostgroups($uri, $authtoken) {
	$data = array(
		'jsonrpc' => "2.0",
		'method' => "hostgroup.get",
		'params' => array(
			'output' => "extend",
			'sortfield' => "name"
		),
		'id' => "2",
		'auth' => $authtoken
	);	
	$response = json_request($uri, $data);	
	return $response['result'];
}

$authtoken = zabbix_auth($uri, $username, $password);
expand_arr(zabbix_get_hostgroups($uri, $authtoken));

?>

If everything worked, the scripts output should look something like this:

0:
groupid: 5
name: Discovered Hosts
internal: 1

1:
groupid: 2
name: Linux Servers
internal: 0

2:
groupid: 7
name: NAS
internal: 0

3:
groupid: 6
name: Routers
internal: 0

4:
groupid: 3
name: Windows Servers
internal: 0

5:
groupid: 4
name: Zabbix Servers
internal: 0

Important
Authentication method is user.authenticate and NOT user.login as mentioned in the manual.

Setting up a Zabbix user with API access

Additional info
http://www.zabbix.com/documentation/1.8/api/getting_started