AWS从PHP访问VPC中的RDS

I am on AWS with a lamp stack connecting to RDS.
I am able to connect from MySQL Workbench using SSH to RDS through amazons security groups.
But ... now that i need to connect from the outside world using PHP, I seem to be having some issues.

Normally i would connect to RDS using sqli like this:

$dbName = "mydb.xxxxxxxxxx.us-xxxx-1.rds.amazonaws.com";
$dbTable = "mytable";
$dbUser = "myusername";
$dbPass = "xxxxxxx";

But since the new instance is on a VPC on RDS public accessibility is not available.

I changed $dbName = 'xx.xxx.xx.xxx:3306' which is the public ip of my EC2 instance, but i get an error: mysqli_real_connect(): (HY000/2005): Unknown MySQL server host 'xx.xxx.xx.xxx:3306'

I am following this guide but seem to be unsuccessful at connecting.

I think I am not doing this part correctly

Create a second VPC security group (for example, "sg-dbsrv1") and create a new rule by specifying the VPC security group you created in step 1 ("sg-appsrv1") as the source.

Any help would be very appreciated, thank you.