PHP mySQLi connection to AWS RDS in AWS EC2 Instance

I am having a problem with connecting to my RDS via the PHP mySQLi connect function. I have established the connection through mySQL Workbench and through a remote connection in the UNIX terminal, but cannot connect through the php code within my index file which is on the EC2 Instance. I have triple checked everything and have added the extension=mysqli.iso within the php.ini file nothing has worked so far. Is there a security group I am missing that needs to be added?