How to connect PHP to MySQL in private subnet VPC

Hi I have two EC2 instances, one in a public subnet and one in a private subnet in my VPC.

Web server (EC2) - public subnet
MySQL database (EC2) - private subnet

I am not using AWS RDS for the database, I am using EC2.

Here is my PHP code:

define ("DB_HOST", "");
define ("DB_USER", "dbuser");
define ("DB_PASS",'dbpassword');
define ("DB_NAME","dbname");
try {
    $dsn = "mysql:dbname=".DB_NAME.";host=".DB_HOST;
    $dbh = new PDO($dsn, DB_USER, DB_PASS);
    return $dbh;
} catch (PDOException $e) {
    echo 'Connection failed: ' . $e->getMessage();
}

What do I put in DB_HOST if the EC2 instance is on a private subnet?

#php #mysql #aws