Validate Facebook Auth-Token. This helper function checks if the Facebook auth-token is valid, and belongs to the said facebook-id

validate_fb_auth_token.php

<?php
// A helper function to validate Facebook Auth-Token.
// This checks if the Facebook auth-token is valid, and belongs to the said facebook-id 

	function is_facebook_auth_valid($facebook_token, $facebook_id)
	{
		$ch = curl_init();
		$url="https://graph.facebook.com/me?access_token=".$facebook_token;
		curl_setopt($ch, CURLOPT_URL,$url );
		curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
		curl_setopt($ch, CURLOPT_USERAGENT, $_SERVER['HTTP_USER_AGENT']);
		// The following ensures SSL always works. A little detail:
		// SSL does two things at once:
		//  1. it encrypts communication
		//  2. it ensures the target party is who it claims to be.
		// In short, if the following code is allowed, CURL won't check if the 
		// certificate is known and valid, however, it still encrypts communication.
		curl_setopt($ch,CURLOPT_HTTPAUTH,CURLAUTH_ANY);
		curl_setopt($ch,CURLOPT_SSL_VERIFYPEER,false);
		$result = curl_exec($ch);
		curl_close($ch);
		$fb_profile = json_decode( $result, true);
		if( isset($fb_profile['error'] ) )
		{
			return false;
		}
		elseif( isset($fb_profile['id']) && $fb_profile['id'] != $facebook_id )//Check if this auth token has the same ID sent.
		{
			return false;				
		}

		return true;
	}