Woocommerce subscription hook after renewal

I’m very new to WordPress and PHP, so I’ve been reading a ton and trying to absorb as much as I can. I know I’m quite close with what I want to achieve but not quite there.

So I have a web store where users can purchase subscriptions (1 month, 6 months, 1 year). My store uses Woocommerce Subscriptions v2.0.8. What I want to achieve is when a subscription is renewed, I want to update a separate mysql database with some information. Basically my database has a list of all active subscriptions, with their subscription id and days left. I believe this needs to be done through actions and hooks. My function is below

add_action( 'processed_subscription_payment', 'updatedays', 10, 1 );

function updatedays(string $subscription_id)
{
	//gets product_id after the underscore
	$productid = substr($subscription_id, strpos($subscription_id, "_") + 1);  

	$days = 0;

	if ($productid == 126){
		$days = 1;
	}else if ($productid == 49){
		$days = 30;
	}else if ($productid == 54){
		$days = 183;
	}else if ($productid == 59){
		$days = 365;
	}

	$servername = "localhost";
	$username = "root";
	$password = "pass";
	$dbname = "database";

	// Create connection
	$conn = new mysqli($servername, $username, $password, $dbname);
	// Check connection
	if ($conn->connect_error) {
		die("Connection failed: " . $conn->connect_error);
	} 

	$sql = "UPDATE users SET days_left= '$days' WHERE subscription_key = '$subscription_id'";

	if ($conn->query($sql) === TRUE) {
		echo "Record updated successfully";
	} else {
		echo "Error updating record: " . $conn->error;
	}

	$conn->close();
}

So my function will accept the subscription key which is made up of the order id + product id. I split this so I can do some work with the product id. Depending on the product, we will set a specific number of days left. Afterwards I want to update the database with this number. The function on it’s own works correctly if I pass a predefined subscription id. I placed the above in my theme’s functions.php file.

You can also see I have my add_action which hooks into the processed_subscription_payment. However, in order to pass parameters to the function, I need to have a do_action which will take the parameter.

do_action('processed_subscription_payment', $subscription_id);

I do not know where to place this line. I tried adding it as well to the functions.php using a predefined subscription_id as a parameter which I know exists in my database, but then the function runs every time I refresh my site’s pages, rather than only when a subscription is processed.

And like I mentioned, I was testing with a predefined sub_id, I do not know how to pass the actual subscription_id of the renewed subscription.

I came across this to get the subscription id

https://docs.woothemes.com/document/subscriptions/develop/functions/management-functions/

but it does not show how to integrate it into my code.

Any help is appreciated!

[ No bumping please. ]