How to verify in-app purchases from AppStore and Market in PHP code (server-side)

verify_appstore_in_app.php

function verify_app_store_in_app($receipt, $is_sandbox) 
{
	//$sandbox should be TRUE if you want to test against itunes sandbox servers
	if ($is_sandbox)
		$verify_host = "ssl://sandbox.itunes.apple.com";
	else
		$verify_host = "ssl://buy.itunes.apple.com";
	
	$json='{"receipt-data" : "'.$receipt.'" }';
	//opening socket to itunes
	$fp = fsockopen ($verify_host, 443, $errno, $errstr, 30);
	if (!$fp) 
	{
		// HTTP ERROR
		return false;
	} 
	else
	{ 
		//iTune's request url is /verifyReceipt     
		$header = "POST /verifyReceipt HTTP/1.0\r\n";
		$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
		$header .= "Content-Length: " . strlen($json) . "\r\n\r\n";
		fputs ($fp, $header . $json);
		$res = '';
		while (!feof($fp)) 
		{
			$step_res = fgets ($fp, 1024);
			$res = $res . $step_res;
		}
		fclose ($fp);
		//taking the JSON response
		$json_source = substr($res, stripos($res, "\r\n\r\n{") + 4);
		//decoding
		$app_store_response_map = json_decode($json_source);
		$app_store_response_status = $app_store_response_map->{'status'};
		if ($app_store_response_status == 0)//eithr OK or expired and needs to synch
		{
			//here are some fields from the json, btw.
			$json_receipt = $app_store_response_map->{'receipt'};
			$transaction_id = $json_receipt->{'transaction_id'};
			$original_transaction_id = $json_receipt->{'original_transaction_id'};
			$json_latest_receipt = $app_store_response_map->{'latest_receipt_info'};
			return true;
		}
		else
		{
			return false;
		}
	}
}

verify_market_in_app.php

function verify_market_in_app($signed_data, $signature, $public_key_base64) 
{
	$key =	"-----BEGIN PUBLIC KEY-----\n".
		chunk_split($public_key_base64, 64,"\n").
		'-----END PUBLIC KEY-----';   
	//using PHP to create an RSA key
	$key = openssl_get_publickey($key);
	//$signature should be in binary format, but it comes as BASE64. 
	//So, I'll convert it.
	$signature = base64_decode($signature);   
	//using PHP's native support to verify the signature
	$result = openssl_verify(
			$signed_data,
			$signature,
			$key,
			OPENSSL_ALGO_SHA1);
	if (0 === $result) 
	{
		return false;
	}
	else if (1 !== $result)
	{
		return false;
	}
	else 
	{
		return true;
	}
} 

please help, what does "$signature" mean? Please explain, it would be better if you share a working copy of code with all param values.

signature is the attached signature of the order.. really long base64 string.. and signed_data is the orders 'receipt' field JSON looks like:
{"orderId":"GPA.3340-1993-0359-####","packageName":"com.###.###","productId":"## ....... }