Find mutual friends of your friends.

facebook-friends.php

<?php

class FacebookAPI
{
	protected $_token;

	public function __construct($token)
	{
		$this->_token = $token;
	}

	public function friends($id)
	{
		 $this->_requestPaged('https://graph.facebook.com/' . $id . '/friends?format=json');
	}

	protected function _request($url)
	{
		$response = file_get_contents($url . '&access_token=' . $this->_token);

		if (!$response)
			throw new Exception("Request to {$url} failed miserably");

		return json_decode($response);
	}

	protected function _requestPaged($url)
	{
		$data = array();

		// while there are still more pages to fetch..
		while ($url)
		{
			$response = $this->_request($url);

			// if there is no more data, stop.
			if (!count($response->data))
				break;

			$data = array_merge($data, $response->data);

			$url = isset($response->paging, $response->paging->next)
				? $response->paging->next
				: null;
		}

		return $data;
	}
}

function find_mutual_friends($users)
{
	$friends = array();

	$facebook = new FacebookAPI('...');

	foreach ($users as $user)
	{
		foreach ($facebook->friends($user) as $friend)
		{
			if (isset($friends[$friend->id]))
				$friends[$friend->id]->count++;
			else
				$friends[$friend->id] = build_friend($friend);
		}
	}
	
	// sort on how many mutual friends he has.
	usort($friends, function($a, $b) {
		return compare_int($a->count, $b->count);
	});

	return $friends;
}

function compare_int($a, $b)
{
	if ($a == $b)
		return 0;
	else
		return $a > $b ? 1 : -1;
}

function build_friend($friend)
{
	$friend->count = 1;
	return $friend;
}

var_dump(find_mutual_friends(array(...)));