<?php
//...

// This will usually be received from an env variable, for example $connectionString = getenv('DB_URL');
$connectionString = 'mysql://user:password@foo-bar.rds.amazonaws.com:3306/database';

// The values below will be your default values if $connectionString is empty.
$dbConfig = array_merge(
    [
      'host' => 'localhost',
      'port' => 3306,
      'user' => 'root',
      'pass' => '',
      'path' => 'wp', // DB_NAME
    ],
    array_filter(parse_url($connectionString))
);

/** The name of the database for WordPress */
define('DB_NAME', ltrim($dbConfig['path'], '/'));

/** MySQL database username */
define('DB_USER', $dbConfig['user']);

/** MySQL database username */
define('DB_PASSWORD', $dbConfig['pass']);

/** MySQL hostname */
define('DB_HOST', "{$dbConfig['host']}:{$dbConfig['port']}");

//...