Fangate class for facebook.

facebook_fangate.php

<?php

/**
 * Facebook fangate class
 * 
 * @example
 * $fangate = new Facebook_Fangate();
 * $fangate->setSecret( $your_application_secret );
 * $fangate->parse();
 * $fangate->isLiked(); // true if the page is liked
 */
class Facebook_Fangate {

	private $secret = null;
	private $data = array();
	
	public function __construct( $secret = null ){
		$this->setSecret( $secret );
	}

	/**
	 * Set application secret
	 * @param String secret
	 * @return Facebook_Fangate this
	 */
	public function setSecret( $secret ){
		$this->secret = (string) $secret ;
		return $this;
	}

	/**
	 * Get data ( after parsed )
	 * @return Array data
	 */
	public function getData(){
		return $this->data;
	}

	/**
	 * Get value from the array
	 * if $data is empty, use $this->data
	 * @param String key
	 * @param Array data
	 * @return String Value | null
	 */
	private function getValue( $key, $data = null ){
		$data = is_null( $data ) ? (array) $this->data : (array) $data ;
		if( array_key_exists( $key, $data ) ){
			return $data[$key];
		}
		return null;
	}

	/**
	 * parse the signed_request from facebook
	 * if $signed_requst is empty, use $_REQUEST["signed_request"]
	 * @param String signed_requst
	 * @return Facebook_Fangate this
	 */
	public function parse( $signed_request = null ){
		if( empty( $signed_request ) ){
			if( array_key_exists( "signed_request", $_REQUEST ) ){
				$signed_request = $_REQUEST["signed_request"];
			} else {
				throw new Exception( "Signed request is empty." );
			}
		}
		list( $encoded_sig, $payload ) = explode( ".", $signed_request, 2 );
		$sig = $this->base64_url_decode( $encoded_sig );
		$this->data = json_decode( $this->base64_url_decode($payload), true );
		if( strtoupper( $this->getValue( "algorithm" ) !== "HMAC-SHA256" ) ){
			throw new Exception( "Algorithm is invalid." );
		}
		$expected_sig = hash_hmac( "sha256", $payload, $this->secret, $raw=true );
		if( $sig !== $expected_sig ) {
			throw new Exception( "Signature is invalid." );
		}
		return $this;
	}

	/**
	 * Get the page is liked or not
	 * @return Boolean liked
	 */
	public function isLiked(){
		if( $page = $this->getValue( "page" ) ){
			if( $this->getValue( "liked", $page ) ){
				return true;
			}
		}
		return false;
	}

	/**
	 * url base64 decode 
	 * @param String input
	 */
	private function base64_url_decode( $input ){
		return base64_decode(strtr($input, '-_', '+/'));
	}

}